well i answer in random order. my battery power has been great. i get 10% an hour. and that's with heavy use of the browser, sometimes games, or whatever. so if you do the math, at 100% i get between 9 to 10 + hours on a full charge, which is great. i have manage to go 3 days without a charge, but of course not with heavy use.
as for the xoom you will get or should get. i would say get the wifi version. that's my device of choice. and i side with you on the point that both your phone and xoom will not be far from each other. so you can always share your current data plan. i sadly have an WP7 HTC HD7 with is not yet ready to share, but with time we'll ge there. now if you are getting it as a gift and the person don't mind the extra $200, then it won;t hurt to get the 3G version without contract. becasue the contract will run you close to half the cost in 2 years. so you have two choices. get the 3G Xoom without contract for $800, or get the Wifi only for $600 and share your current data.
Note that if you do opt for the 3G xoom, you cannot use another carrier sim card (at this time) like say t-mobile, or at&t. so you best bet will be the wifi, but then again even the 3G can work as a wifi, and in time the 3G will be crack to work with other carriers, cuz this is Linux after all. but the choice is yours. i someone was buying me a gift, and money was not an issue, i would get the best there is, which in this case is the 3G Xoom. plus if you wanna get a few add ons, like case, hdmi cable, keyboard, and like me the up and coming outterbox defender series case for my xoom, then the extra $200 can go to better use. so the choice is urs bro, doesn't seem like a hard thing to settle on. i think i cover all the basics.