You can still continue to use XNA if you like it - It won't stop working or refuse to install on people's computers, it's just not going to be developed any further. If you want to get it onto Windows 8 or any other unsupported platform, you could use MonoGame to port it over when you need to.
For MonoGame, you just need to use XNA's content pipeline to compile your assets before adding them to your project. There's also a Content Compiler project on CodePlex that might simplify things, but I haven't tried it yet.
If you're comfortable using those frameworks, and enjoy it, there's no harm in continuing to do so. Any concepts you learn in one language usually transfer to a new one, so you're not going to "waste" any time now even if you change later: When you're certain you'd rather be working with C++, and feel you're ready, you can do so whenever you want.
I hate to 'leave it up to you' since you specifically asked to avoid that, but I don't know that anyone's opinions would be relevant to your decision, the big picture is the one you're painting yourself. I could advise you to go with C++, just because that's what Epic used for the Unreal engine, but that's irrelevant if you have no interest in working there. (As well as a poor basis for advice in the first place!)
Basically, you got caught up in having to make an unexpected decision, while feeling like you don't have enough knowledge to do so. It's not as dire as it seems, and truthfully in the longterm, you'll be fine no matter which way you go. It's really only a matter of which feels more comfortable for you.