It really comes down to what you do and what do you need. I thought about getting a Mac Pro, just got a nice new job so I could have afforded it, but I realized I did not need that much, I had a Power Mac G4, but for what I was doing which was using it as a DVR with EyeTV 3, it was getting wayyyy to slow for editing out commericals.
So I got a used Power Mac G5 Dual 1.8 Ghz, the model with 8 ram slots, not the more common 4 slot one, for 600 bucks. *Side Note* Interesting enough right after I got my Power Mac G5, I found a program that could mark commericals in EyeTV3 automatically! *End Side Note* That Power Mac does everything I want and more. Yeah it is older equipment, and that model you can only put 2 hard drives in unless you get something like the Sonnet Jive, but coming from a video perspective, I love my Power Mac G5, I can be recording a tv show, marking commericals in a recording, encoding an edited video AND watching a finished video in iTunes all at the same time without skipping a beat. The only thing I did was add 2gb of ram so now I have 2.5gb total, and a 500gb Seagate SATA hard drive. I spent about 770 bucks and this computer can do everything I want.
Now compare that to the Mac Pro. If I bought one the cheapest I could right now it would cost 2299.00. I am not saying its a bad computer or anything. My Dad bought one last year! HAHAHA it was so funny, he went from a Power Mac Dual G4 450Mhz to a 2x Dual Core Xeon Processor Mac Pro. It was so funny, my dad said "I can't go make a pot of coffee anymore waiting for photoshop to open . . . " that one still cracks me up.
Anyways, I degress, if you have the money and the need for the Mac Pro, then get it, however if a good used Power Mac G5 will do what you want it to get a used Power Mac G5 instead. Same thing goes for the MacBook Pro, see if you get a deal on eBay or something instead of buying on brand new.
As a general rule, I never buy a computer brand new, I did twice, but hey I don't think I will do it again, I could have saided money if I had waited a few months and gotten on used.
Even with all that said, it really comes down to personal preference. My Dad spent big bucks on a Mac Pro to upgrade from his Power Mac Dual G4 450Mhz which he desperately needed being an Profressional Photographer. I tried to get him a used Power Mac G5, like the Dual 2.7Ghz, but he just refused to buy a used Mac.
Thats my two cents . . . well alot of two cents!
-iGrant