Sanke without a doubt IMHO! There is a reason the commercial breweries use them, IME they work very well! The sanke keg coupler once locked on the keg is just about bomb proof. I have never read of someone ending up w/ gallons of beer in the bottom of the keezer due to a leaking sanke keg.
For tips on reinstalling the spiral retaing ring try this...
the majority here uses corny kegs...so when the question is posed, the answer is somewhat predetermined I would guess. Of course opinions and mileage vary....cheers.
Also, sanke kegs come in 1/4 B size, 7.75 gal tall quarters which is a nice size IMO...a bit bigger yet not so heavy you need a crane.
Regarding replacing the sanke o-ring after each disassembly, I have never heard this before? I guess this could hold true for any o-ring on any keg...sure I guess this would be best practice on all kegs, sanke or corny, but I have never done this w/ a sanke keg.
The only downside I have found w/ sanke kegs is that dry hopping in the keg is a bit challenging, I have recently been using long narrow hop bags that will fit through the opening.