I think the answer comes from your budget.
Since you do 3D/CAD a quad-core system is going to be much better.
So is a screen with higher resolution.
The 15" MBP's res is not as nice as a 1920x1200 iMac, let a lone the rumored 28" iMac.
Don't know what 3D package you use, but e.g. Maya has a recommended screen resolution of 1600x1200 to guaranteed show all menu items and onscreen controls. A 15" MBP's resolution does NOT cut it. You'll still be able to use it, but it's outside the recommended configuration and some features might be hard to access.
It is pretty clear though that new iMacs should happen within 4-6 weeks from today. My money would be just before the Apple Shareholder meeting mid February. Apple so far very often had some sort of release before big shareholder meetings.
Last year we had a really good January and February, with something new almost every Tuesday. Could be happening again this year and the iMacs could be introduced much sooner. Although I do not expect many releases around President's Day. That event will just drown every publicity.
To answer the question:
- If you can afford a quad-core iMac (likely a bit more expensive than current models), or
- If you can afford a 28" iMac (also more expensive than current models)
Then I would wait 4-6 weeks for these.
If they don't happen, by that time the 17" MBP should be in stock and you might have saved up even more to afford it?
If you cannot afford any of these bigger, better iMacs, then there is not much point in waiting. You might as well get the MBP now. Unless the screen resolution is a dealbreaker for you. 1920x1200 is much nicer than 1440 by 900 for 3D work (assuming you're thinking of a 24" iMac).