Just to correct/clarify...the 45 Colt MG should be a model 625...the only 629 MG is 44 mag.

I have both, and don't think you can go wrong with either. It depends on a lot of factors...do you already reload or have ammo for either (as Taroman pointed out)? What do intend to use it for? For typical factory loads, I find the 45 Colt a little more comfortable to shoot in the relatively light-framed MG. Obviously 45 Colt can be loaded pretty hot too, but 44 mag will really buck at the hotter loads. The 629 has the advantage of being able to shoot 44 specials if so inclined (though those are not cheap to buy if you are not reloading them). I wouldn't want to part with either, but if forced to pick just one it would probably be the 625 45 Colt, since I have other 44 mags that are a little heavier and easier to shoot.
Brad