I'm on my 3rd 3Gs (16) in 3 days - - the 1st one had the same wifi signal strength changes (I had it for 2 months), and only when I stopped in an Apple store this weekend did I see a full 3 bar strength indicated . . . explained my situation and was told it was probably the hardware . . . swapped it out, and couldn't even get 3 bars on the new 3Gs in the Apple store (was told not to worry, as long as it didn't drop to 1 bar) . . . got it home, same problems (dropping to 1 bar even only 2 feet from my Netgear router) . . . swapped again today (although both phone no. 2 and no. 3 worked at full strength in a different Apple store), got it home and same problems - - starts at 3 bars drops almost immediately if I check email or do anything else to 2 and then 1 (and then stays at 1 bar), even only 8 feet from the router . . . did all the Applecare suggestions too, including rebooting, reinstalling, etc., with the same problems between phones 2 and 3 . . . I lot of time and hassle, with Apple reps. saying "they've not seen this problem" and "it must be my router?!"
says to switch to wifi if available as 3G uses more battery life. I'd say the difference is minimal, if any.