Like Stan said, really no difference in strength at all. The hex being older will typically bring a bit more $$
I like the hex just because they will often have the older Russian markings as well as the Soviet markings. There are also different round recievers depending on when built (e.g. pre-war and during WWII)
Check out 7.62x54.net for a good run down of all the various types and yearly production numbers. Buy one and go from there. Mosins are pretty addicting.
As with any milsurp, look for a good bore. Bluing on most Mosins is surprisingly good. The stocks can be easily touched up with some 0000 steel wool and thinly mixed shellac.