I'm assuming both requesters are going to be useing FCP or FCE at least.
If your going to be using Motion, then you definitely want a dedicated graphics card. The better the card, the better the performance. Also the realtime playback effects of FCP use the video card. I'm assuming FCE has that feature too. For the Mac Pro buyer this means you may want to upgrade the card when you buy, to the ATI Radeon X1900 XT 512MB card. For the iMac user it means you should NOT buy the lowest end iMac or a Mac mini or a non-Pro Macbook.
The current iMacs of choice use the ATI Radeon X1600 graphics processor as where the new MBPs use NVIDIA GeForce 8600M GT graphics processor. I haven't read performance numbers, but I would expect the MBP has a better processor. The older MBPs used the X1600. Oh, and the more memory the video card has, the better. Remember, you cannot update the graphics system in non-Mac Pro machines.
The last iMac release was in September, so some people are expecting a newer version real-soon-now. Perhaps that would have a newer faster graphics card. Don't ask when because Apple is the only one who knows.
To save dollars or better spread you funds, look into Apples refurbs. For the fixed systems such as MBPs, you want to understand the processor lineups too.