As you've noticed, :), all of those engines have their benefits and weaknesses. I would suggest you consider, as a major decision maker, whether or not the engine you choose has a way to export models from popular 3D modeling packages. You have to author your content somehow, don't you? Hand-coding C/C++/VB code for every 3D model assets will be prohibitively expensive.

Newton is integrated with 3D Game Studio, which I know nothing about. NovodeX has an exporter (Flexporter actually) for discreet 3ds max 5 (NOT version 6, and NOT gmax) and Milkshape. As far as I know, there are not readily available exporters for ODE or Tokamak. NovodeX has a solid, full time team of developers behind it, which surely would be a benefit. But, I also know the author of ODE to a knowledgable, reliable developer.

In reality, if you can be flexible about the exporting, I would say one of your first tasks on the project should be to do a short evaluation of the different engines. Create a simple problem such as your game will simulate, and duplicate it in all the engines. Use the same rendering engine for all the demos, so you're comparing apples-to-apples. It would not do to use the demo players that are available with the different engines. Document how long it took you to get the demo up and running for each engine, how fast it runs (frame rate), how bit the executable is, how stable the engine is, your general understanding of the API, etc. Then make a decision from there.

There isn't much happening on the tokamak forum anymore. The developer doesn't show up there anymore, too.
Therefore many tokamak people switched to newton.

A model loader loads content that has already been created and stored to disk in a file. Where did that file come from? The modeling package and an appropriate exporter! Basically, you go into Milkshape or 3ds max, create a 3D model, maybe add physics properties such as mass and constraints, then "export" to a file that is in the format your model loader understands.

The trick is that there are lots of 3D modeling packages that allow you to create 3D models, but those models don't have physidcs properties. To get the properties need to simulate the physics you either have to add the information manually to your code or to a special configuration file, *or* get a plugin and exporter that allows you to add the properties within the 3D modeling package then export to the appropriate file format. A good exporter is a critical part of the game asset pipeline.

Note that the standard game model file formats, such as the Quake formats, might not contain physics properties. And so loaders of these formats won't be sufficient for physics simulation..