I would go with the 600. I have a 600, 2 660s, and 3 XP-100s (same basic action). The 600 and two of the XPs have in excess of 500 rds, and I have not experienced the "cottonball" or accuracy problems referred to in the earlier post. I am not saying that it does not exist, just that I have not experienced it.
If it makes any difference, in the seventies the 788 sold for around $67.50 (same as a Ruger Blackhawk), and the Mohawks went for around $120. Both were considered to be price leader guns. The Mohawks were not as finely finished as the regular 600s, but were not unattractive.
The 788 was touted as ugly but very accurate. Be sure the box magazine is the correct one, and the body, or lips are not dented. FYI, with the 788 being a rear lug lockup, it would probably not be as accurate if you intend to hot load your rounds. This is pretty typical of rear lock up rifles. Neither would be a bad choice.