I have both a 4" and 6" M629. In the past I have owned several others of either flavor. I like them all but prefer the 4" revolvers.
One thing that I notice is that when you are running full house loads (I don't mean monster handloads, just full power factory loads), the recoil in the 4" guns is a whole lot more severe than the 6" guns. For some reason, there seems to be a whole lot more wrist torque with the 4" guns. In my 4" .44 Mag, I like a 240 grain LSWC bullet at app 1000fps for a pleasant load that will get any job I need to do with a handgun done.
I realize that recoil is subjective and what bothers me may not bother someone else but that is my $.02 on the subject. I don't know if that will factor in on your decision.
Another issue is if you are going to carry it a lot. A 4" revolver carries a whole lot easier than a 6" revolver. It catches less on things, doesn't end up hitting things as often and even in field rigs the 4" just seems to stay out of the way better than a 6" revolver. I would throw into the mix that a 4" N frame of any flavor carries very nicely in an inside the waistband holster provided you buy your pants with enough room, use a good holster and wear a good belt. It won't disappear under an untucked T shirt like a J frame will but under most covering garments, it does well. Frankly I also like IWB carry in the field where I don't have to worry about concealment. I am right hand dominant by training, left eye dominant to shoot. While I hold a handgun in my right hand as the primary hand, I shoot rifles off my left shoulder. I also tend to carry a rifle slung on my right shoulder. By using IWB holsters with 4" N frames, it keeps the rifle and the handgun from getting tangled up. Can't do that with a 6" revolver. Assuming no cross eye / hand dominance that wouldn't be a factor for you but thought I would throw it into the mix because I am not the only one around with this issue.
My best advise is see if you have any buddies who have both that you can shoot before you decide which one to buy.
Chet the older guns are as strong as the new ones.
