Of course the Mac Pro would be faster, run cooler, and have way more space for drives, etc. So if you need the portability, then you need the laptop, but if not, maybe the Mac Pro is a better idea.
I second that. In my work, a Mac Pro (four-core with 8 GB RAM} is pleasantly and noticeably faster than a MacBook Pro (large 7200 RPM drive and 4 GB RAM) for Lightroom and Photoshop. For performance, flexibility and the ability to handle a large collection of image files, the Mac Pro wins.
If portability is important, you can make do with a MacBook by adding external drives (esata using an express card adapter gives best performance). Although not officially supported and a bit expensive, you can run 6 GB RAM in a MacBook. But, do you need a 17" computer? As a hobbyist, you aren't visiting clients and trying to wow them. A 17" MacBook is big and heavy. If most of your work is at home with an external monitor, you probably would be happy with a 15" MacBook. Same performance, less money, (but less of a status symbol ).
Read this
opinionated guide to configuring a Mac Pro and a MacBook Pro for photo use. (In my opinion, more than 4 cores and more than 8 GB RAM is too far into diminishing returns to fit my budget.)
What works for me is a small notebook for field use (a 13" MacBook or a cheap PC, plus an extra drive; light, compact, not a huge loss if stolen or damaged) and a carefully configured middle of the road Mac Pro. I don't do serious processing in the field, just backup, review, adding metadata.
If buying anything Mac, the factory refurbished machines you can get directly from Apple are terrific value, fully warranted, and indistinguishable from "new." Also, for best value, buy the minimum configuration from Apple and add third party memory, drives, and monitors.
Cheers.
