If the software is computation-intensive and available for Windows only, you really should buy a Windows PC to run it best. With a Mac you would pay more and it probably wouldn't work quite as well (although it might work fine).
I'm not familiar with architecture apps, but read the system requirements carefully. If they have a Mac version, check what it says about the graphics card. If any card is fine, a MacBook Pro would be nice for the portability. But if the graphics card matters, the iMac will have more powerful options and a bigger display. If you got a MacBook Pro, you would probably want to also budget for an external monitor to plug in. Graphics apps tend to need a lot of room to spread out your toolbars and palettes.
Also, if you choose an iMac, remember that the 21.5-inch has no expandability and the RAM amount is more limited. This may affect your work. Again, read the system requirements for your CAD software. The 27" is the one to get if you can afford it.
I agree the Mac is good for some many things especially for consumers and light business needs, ranked very highly but it's not for everyone and perhaps for what you require a more specialized Windows machine is better suited.
Windows 7 will get support until 2020, Apple tends to upgrade their OS X versions annually, so that's a slight drawback for someone who needs stability in their software amd investment.
The only somewhat 3D capable laptop Mac is the 15" MacBook Pro and it's video card isn't as powerful as PC models for specialized professional needs. Neither is the one in iMac for that matter as Apple customers have more or less gone for thinness, style and lightness verses uber performance, which mean thicker and heavier.
The MacPro hasn't seen a update in two years, PC's with Xeons have outstripped it for performance. Apple recent issues a press statement it's being pulled from European markets for not updating to meet new EU requirements (set nearly three years ago.) So perhaps a powerful Mac tower is out and a PC one with Windows 7 is better suited to your professional needs at this time.
Also then comes the operating system and programs tailored for it, which you really would have to run Windows 7 on the Mac in Bootcamp, but may have issues etc for hardware compatability also problems with tweaking things and custom drivers etc., as Mac's are not generic PC's although they can run Windows 7 in BootCamp
I can make some suggestions for some uber PC machines for awesome professional results.
HP has a line of Elitebook 8770w Mobile Workstations with some truely awesome video cards for fast rendering speeds costing upwards anywhere up to about $6500 if you so need it, plus the displays are larger and detailed.
Sager has some awesome machines anywhere from $2000-$5000
Dell has their Alienware series, but that doesn't sound so professional
Asus has their The Republic of Gamers line of powerful prebuilt desktops and laptops, but they all don't look like 3D gamers machines.
Not exactly as nice as a Mac for the thinness of hardware, but they are performance powerhouses for specialized needs, and they should come (request it) with Windows 7 Professional (so you can run XP programs too).
Windows 7 is not a bad operating system, everyone seems to love it, even Mac users like it.
It does need the services of a professional Windows geek to make sure it's crapware free and anti-virus installed, System Restore Disks and images made before being used online. I also suggest Deep Freeze if you really want a almost trouble free PC, a simple reboot resets the machine back minus any changes.
I hope this assists and perhaps you will consider getting a Mac or iPad for less intensive requirements.
He had seen HP, Dell, alienware thought there were good, but never heard Sager: D, Sager wonder this well appointed affordable ah, did not know much of Sager find out more, so I decide, because there was only observed Asus and HP with good performance and good appearance.
If for now I'll stay with my ipad2 to my moments of relaxation , I wanted to increase the apple family, but for now you have to wait.
Thank you very much, very nice of you for such useful information.