Is it so expensive to have both in one device? I defiantly prefer WiFi but 3G is useful in cases when you are outdoors and need some information badly. Like when the movie starts or where my favorite band is playing tonight. I use my android phone all the time during Friday nights to decide where to go next or to show funny YouTube clip I found. Still I don't know if I would bring tablet with me in such ocations if I would have one. On other hand, I've used phone as car gps a lot of times and then tabled would more handful.
For those rare cases when I need WiFi to download large files and I don't have such luxury I wrote small app for that. That simply remembers list of files to download and when WiFi is available it downloads them. It's in the market but I won't give the link here due to ethical reasons.