The Kaiser will be the best pocketable HTC device out at the time of release. It has the same CPU and HD space as the Mogul, but has twice the RAM, WM6, GPS, better camera and a tilting screen. Probably better build quality as well, since it uses the same metallic casing as the Herald.
3G will drain the battery just as any other high-speed network connection for your phone. You likely won't have any complaints as HTC has great batteries. AT&T should be getting the Kaiser in as the 8925. I'm not sure how much they'll screw up the ROM, however.