I've used EAI, Surly, and Soma cogs. I haven't been able to detect a significant difference in performance between any of these. All of them are good quality, as are Duraace and PhilWood cogs. The most important thing is not to cheap out and get a no-name or low quality cog...too many horror stories about these stripping your hub.
As for gearing, thats a personal choice. If you are going with a free/fixed flip flop setup, its typical to gear the fixed side a little higher because you have to pedal down hills.