I was in the same boat as you... however, since Apple will seemingly never release a powerful laptop, the tower looks like the way to go. That last statement was partially in jest... but only partially (come on Apple, get the lead out!).
In looking at the future (as much as is possible when discussing technology), it seems to me that a computer with an upgradable
GPU is fairly essential. As of now, the high-end video cards from NVIDIA and ATI have 256MB VRAM. With more and more processing being pushed on the GPU, video cards will (I'm guessing) become much more essential to overall performance.
Then there's the multi-processor issue: Modul8 is coded to take advantage of this, but it will probably be a cold day before an Apple laptop sees one–this isn't a bash on Apple, I've been a loyal mac user for almost 15 years, and will remain one for the forseeable future–just trying to be realistic.
Then there's the fact that you get WAY more bang for your buck with a tower. For most VJs, portability is a key issue, but I'm in a band, so one more case in a van full of cases is no big deal. However, I'm still keeping my fingers crossed the next wave of Powerbooks will have a significant performance boost.