I never had any problems with the 7700 cogging. I ran mine on up to 3S lipo. The real drawback that I could see with the 7700 is that it doesn't take gearing as well as the 5700.

You are on the right track though. 7700 for 2S and nimh, 5700 for 3S, and the 4600 for 4S. I've found the 4600 to be the most versatile. You can go from mild and sedate to "what was that blur" speed. The 5700 is also pretty versatile, but just not as much as the 4600. If you want something to run now with the nimh cells and then be able to upgrade later with lipo, just go with the 5700. It is THE middle of the road choice.

Now that I look back on all of my MM purchases, I could probably have done without the 7700. The 7700 only really shines in an on road car or vehicle that has very limited battery space; ie, something that requires running relatively low voltage.