For carry, a 3" is always better, and the 3" S&W 65 is a sought after revolver. $500 is a good price for one if it is in very nice condition. A scuffed up one should be high $300. They're not the best for shooting lots of .357, though, and the Rugers you mentioned were built for .357 so fire away. I've owned many S&W k frames but am down to only one, a 4" 13. On several occasions, I have called and emailed S&W and asked about shooting magnum rounds in the K frame .357's. Every time, I was advised to shoot only 158 grain .357 and limit those to familiararity only, while practicing with .38. The do not make barrels anymore for K frame magnums, so if you split a forcing cone, you're out of luck.
On the other hand, the Six series is no longer made, so you won't be able to get parts for those, either. But the Six series are renowned for strength and the odds are very small you will ever need repairs on one. Good news is that if you DO need reapirs and Ruger can't do it, they offer you a new GP or SP101 below cost. A guy on here sent in one of his Six revolvers and they couldn't repair it, so they sold him a new SP101 shippd right to his door for like $340. S&W won't do that for you.
Bottom line, if the 65 is in good shape and you don't intend to shoot lots of 110 or 125 grain .357 mag rounds, that is the one to get.