I had the Transformer, Loved it, but had to send it back because of some hardware issues with the volume and power buttons. I was all set to get another one, but decided to go with the Tab instead.
The Transformer is a great device, and defiantly has more versatility. (more ports, SD Card, and the dock) If you see yourself getting an external keyboard, the transformer would most likely be your best bet because it makes the battery life insane.
The Tformer is heavier than the tab, but you won't notice this unless you go and use someone's tab and then switch back. It's not "Heavy" just heavier. The screen on it is great, not as good as the Tab, but it's not horrible either (and they're both a lot better than the Xoom's screen IMO) So if you see yourself using HDMI, or an SD card, or the keypad, go with the xformer.
For me, I didn't really see myself using that stuff. (well I considered the keyboard, and then I discovered Thumb Keyboard). And for me the screen+thinness+the potential that Touchwiz won't suck (the previews actually interest me) were enough for me to justify the extra 100.
Someone mentioned force closes.. the only time I experienced force closes on either device was when I tried running an un-optimized tablet app, or I went to a website that wanted to throw flash ads in my face. (it didn't play well with the on demand option I think). My Samsung had an issue once with
FC when trying to boot up the Samsung account (required for updates) but has been fine since.
TL

R
Get the Transfomer If: You see yourself using the keypad, or you have a known use for the HDMI plug/SD card
Get the tab if: You want the best feeling/looking device, and you're ok with NOT having those ports.