I've just done what you are planning on doing. While I don't have a uMBP, rather a classic MBP -- I decided that I wanted a more portable machine that had a great build quality and look to it. Enter the 2.4 uMB.
It's especially helpful on a college campus because my 15.4 has a tough time fitting on some of the lecture desks. I even can use it on the airplane without discomfort compared to the bigger MBP.
If you are planning on playing games, the 9400M is no slouch, but its certainly better to have a dedicated GPU like the 9600M GT if you are an eye-candy person.
Granted the screen quality isn't the best, but if you look at it dead on, it actually appears somewhat decent.
I really don't care about the difference in resolution between the uMB and the 15.4 uMBP mainly because its just one step up on the 16:10 aspect ratio. If it was the 17" uMBP we were talking about, then thats another league -- and a big difference.
I personally have a WUXGA 24" 8-bit panel display when I need the screen real estate or have to do something with accurate colors. Otherwise the uMB is a great road warrior, and I am able to do light coding on it.
Overall, I have to say I am impressed with the uMB line, it just doesn't feel as consumer as the polycarbonate MBs, and I truly believe that the 2.4 uMB is a true successor to the 12" powerbook.