Nobody mentioned the elephant in the room: your target audience's hardware.
I can safely assume that few,if any, of us modest indies will create AAA quality games so what good is it to have a monster gaming PC (other than build times obv) if your poorly optimised little game runs perfectly on that machine due to its monster performance making up for lack of good programming skills, but won't run smoothly on your user's Walmart promoted crappy laptop?
He asked for a development machine, not a test machine.
We might test on Walmart's laptops, but you'd have to shoot me before I develop on one. 
It might not be what your looking for and a bit overkill but my cousin and I built a mini-itx PC which is pretty nice and very easy to take with you.
It might not be what your looking for and a bit overkill but my cousin and I built a mini-itx PC which is pretty nice and very easy to take with you.
Now if we are starting to talk about "semi-mobile" solutions I just have to mention this:
Yes, its just a barebone without windows, RAM or Disk (though you can get a Plus version with RAM and Disk installed)... and yes, it costs 800$. But it might be the first SFF PC with a real gaming grade GPU. Not that intel iGPUs or Gtx x60m crap... it has a GTX970m builtin. *
Now I do know this is
1) More expensive than a small desktop assembled with desktop parts that would be cheaper and easier to upgrade, and would be faster for the same price
2) Less mobile than a laptop with the same components
But
1) It is MUCH smaller than any Mini-ITX build you could do on your own... if you do not need to chuck your rig in your bag that might not be showstopper, it is if you need to. Not so with this thing.
2) it costs half of what a laptop with similar components cost (save the weedy 15 watt CPU, but more on this later), all while giving you more power because of a better airflow and thus less throttling (at least for the GPU part)... and, to be honest, it is even smaller than your average laptop at 20cm x 20cm
The only real downer is the choice of CPU.... for a GPU this powerful a 45 watt class 4 core mobile CPU would have been great. Sadly you only get the two core 15 watt i5 5200U...
But: in about 1-2 months, Zotac will release a Steam machine based on the same barebone. They already revealed that this Steam Machine will have a quadcore Skylake CPU (which means 45/47 watt range), and will cost around 999$, about the same as the Plus version of the non-SM Barebone. Now THAT sounds like a powerhouse you can chuck in the bag (IF it still doesn't throttle outside of stress tests).
Disclaimer: I am not working for Zotac 
I am just really excited about at least the steam machine version, and a little puzzled about the lukewarm reception both these machines received. I guess they fell in the large pricegap between laptops and selfbuilt Desktop rigs, to mediocre in power to compete with high-end desktops and to expensive to compete with the normel ones...
They might not be a costeffective steam console... but I am still tempted to buy the steam machine version, install Windows on it and use it as "Power Booster" for my Wacom Companion... which is good for drawing or office programs, but just not powerful enough for playing games, using engine editors or 3D modelling apps.
This little thing will add not much bulk to my bag, but would allow me to work on my game prototype everywhere where I find a power outlet to power this thing and enough desk space to set everything up.
I'll see if I have 1000$ to spend at Xmas... most propably not. Still, it is an option for people that do need a new machine.
* (Don't be fooled by amazon calling it a GTX960. Seems like Nvidia now forces all the system builders using mobile GPUs for non-mobile systems to rename the mobile chips to their next lower non-mobile counterpart. So the GTX960m will become a GTX950, the GTX980m becomes a GTX970, and the GTX970m becomes a GTX960. Makes kinda sense because these cards are roughly comparable, still makes things confusing when you suddenly have a "GTX 960 with 3G VRAM" )
When I want to use my Laptop in bed there are several steps to take:
1. Plugin the powercable
2. Plugin the mouse
3. Make place for mouse
4. Plugin Cooling Pad or the 90°C/200°F on my stomach combined with the bedsheet get reeeeeeeeeeeaaaaaaaaaallllyyyyyy uncomfortable
5. Rearrange all cables so it is comfortable
"Portability" is just half of the truth.
I used only a laptop for about 6 years.
Earlier this year I was lured back to a desktop as I realised I didn't actually move my laptop for two years solid.
Some people here mentioned that you won't need the performance to write AAA games but remember you might want to run an AAA quality engine, plus visual studio, plus a Web browser, plus a pixel editing program, plus a 3d package such as blender, and your email client, im client of choice, and all sorts of other stuff, maybe simple sound editing program like audacity. It all soon mounts up.
I started out with eight gigs of ram, and my level and game are huge (its an open world game) and very graphically demanding as I'm using a lot of the facilities offered by unreal engine. As such even as a one man band 8gb wasn't enough unless I wanted to be closing apps and opening others like some edition of windows 3.1. I bit the bullet and upgraded to 16gb and it's now smooth as butter.
The moral? Whatever you choose don't skimp on ram. Future proof and get 16 or even 32gb, and you won't regret it as your pc may still be quite usable even in five years time.
Two cents worth firmly inserted.
It might not be what your looking for and a bit overkill but my cousin and I built a mini-itx PC which is pretty nice and very easy to take with you.
There's something wrong with that list -- You've got a Mini-ITX case and an E-ATX motherboard.
I've moved to a (higher-end) laptop for serious work a few years ago. You pay a premium for portability, but its nice for being able to get out of your home office (maybe to class, maybe to a coffee shop, whatever), and important when you need to show off what you're working on. That said, I'm not fond of working at a small screen (or just one screen), so I have a matching dock and several large monitors, along with a nice keyboard. My lappy is a Lenovo w530 -- 15.6" 1080p screen, Quad i7 2.6Ghz (boost, 2.0Ghz nominal), nVidia Quadro K1000m (with optimus), plenty of RAM (32GB), and a quality SSD (Samsung 840 pro).
I have a higher-end desktop for gaming, which I can always call into service if the laptop comes up short, but so far there's been no need. If you don't need the mobility, a desktop is indeed bigger bang for the buck; that said, for my money, I'm becoming increasingly convinced that a laptop/server combo is the best solution -- I like having a solid home server anyways, so selecting a beefier CPU and additional RAM to handle the occasional load adds only a marginal cost (say $200) -- its fairly easy these days to farm out software compilation and asset builds to these additional cores (and your laptop is probably just fine for incremental builds anyways, unless your code has structural problems) -- you can even add a GPU if your tasks make use of it. If you're so inclined, you can even do most of that server stuff using cloud services, or on-demand (hourly) VPS/Docker instances.
Right now is a fantastic timing to build a desktop since the new skylake intel cores were just released.
But if you absolutely need the portability, I recommend a high end ASUS or MSI gaming laptop.
