The 2100 mah gives you just enough to make it through the day with significant use, certainly so if you're not playing with it all the time. Mileage tends to vary with individual users on battery questions. A 3800 mah battery certainly will last you, but case options are limited.
I hear that on the case issues, even the 2100 has issues with cases being rare. I went out and bought a Seidio specifically for the 2100 mah battery and it fits like a glove and is not too bulky.