I'm not going to compare any IBM with anything else. No way. If you are old enough, you will certainly know what I mean. But I'm taking your advice and will check out the MacPro for my first Mac ever...thanks!
Basically, there are three desktop models: 'mini', 'iMac' and the MacPro.
The MacPro easily has the most CPU power and biggest memory capacity, but needless to say you don't need a dual socket / 12 core Xeon box for office productivity or web browsing. The MacPro is also the only Mac with PCI/PCIe expansion slots and in which you can change the video card. If you need a RAID card you have no choice but the MacPro. The MacPro line hasn't been upgraded in a while, it lacks the 'SandyBridge' generation CPU's and Thunderbolt ports which have by now even found their way into the entry level systems. Evil tongues also say that Apple is making most of it's money with gadgets and the niche of pro workstations just isn't lucrative enough anymore.
Since you're asking for 'my first mac' and not 'the ultimate FinalCut Pro HD workstation', I would advise against the MacPro, just like I would advise against an Onyx3000 as a first SGI
If you want Office productivity & web browsing, a Mini with a quad core CPU is more than adequate. If you want better 3D graphics, the iMac can be had with an ATI GPU.
Then there's the matter of taste: not everybody likes the glossy screens of the iMac for example. Or maybe you bought a 30" EIZO screen last year and want to keep using it. It's all a matter you what you've got and where you want to go (no pun intended), but you didn't tell us your intended use for the system.
Personally, I bought a 13" MacBook Pro as my first Mac. I was looking for something for light browsing duty around the house, portable, but with a keyboard. I also take it with me to offload my Nikon when we're on holidays (I wouldn't be the first who had his camera stolen or memory card die on the last day of my holidays). I'm quite happy with it. Not only does it work very well as a computer, it has something that makes you
to use it. It also makes the Dell laptop it replaced look incredibly clunky.