The battery in the 3G iPad is just fine. I've heard there is ~10% extra battery drain when using 3G networks, but my battery lasts for frickin' ever. I use it iPad throughout the day and I see around 20% drain on the batter per day. So on an average week I can go 4-5 days without charging it up again. By comparison I will totally drain an iPhone 3G in less than one day. Since I have the 3G iPad and am not using the phone as much my iPhone only uses maybe 40% of the batter life in a day.
As for the 16 vs 32 I cannot say. I have the 64gb one and have about 60gb free. But I use mine mostly for email and web work as well as remote administration of some servers. I have a 16gb iPhone and it's totally full so it really depends if you are going to be offloading video/audio to the ipad or not. Personally, I like havign plenty of extra room so that if I take a longer trip I can just use it on the plane or anytime I get bored, in addition to doing actual work. When in doubt... get more memory I say!