My opinion is the 357 is the best of the two by far. While the MK V is fairly rare, the 357 is the better of the two by far. The 357 is made from forgings and handwork as it was done for so many years. This handwork resulted in fine, fine actions and overall pleasing results. Accuracy will be top and it will be a fabulous looking and working revolver.
The MK V is made in the newer method of castings and "sintering" metals. While these can be very functional guns, they lack the "soul" of the earlier guns and will never, ever be as well done as the originals. In my opinion, the MK III, IV and V Colts were just soulless and very poor renditions of the original Colt guns, with emphasis on the revolver.
Its your money, and you asked for opinions. As evidenced by the over-whelming response for the 357, you will have to decide what is you are really wanting. As noted, many of us would have taken the 357 immediamento! Good luck. But.... BUY the 357!