Interesting thread.
I'm in the same boat trying to decide between an iPhone 4G upgrade from a 3Gs and an iPad 3G.
I was planing on waiting for the iPhone 5G (or whatever it's called) since my contract won't expire for another year and get the iPad now. However, after seeing the stunning Retina display on the iPhone 4G, I am re-evaluating which way to go.
Don't get me wrong, the new display is very nice, but I wouldn't consider it a must-have if your 3GS is doing the job. The gyroscope has potential, but time will tell how well it is used and how many must-have apps require it. If you were eligible for a full upgrade, there would be little room for debate, but under the circumstances, I think that your 3GS is enough for now.
The iPad is a little faster in most typical operations because its A4 processor is not underclocked like it is on the iPhone 4. The larger screen lends itself well to applications with spectators (showing a movie or playing something like air hockey) and "lazier" consumption, that which doesn't have the screen pressed against your nose. If you are just looking for a more powerful device, waiting a little longer for the next iPod Touch might be more up your alley. If you don't have an idea specifically for the iPad, it's quite a premium (greater cost, less pocketable) that may end up going to waste.