While I hunt deer with, reload for, and hunt with a 257 Roberts (Winchester featherweight) and my son a 260 Rem. (Model 7) which I also load for, I guess I will give an opinion or a statement or two. There are pros and cons on both albiet, quite paltry at best. Go down and read the post on the 6.5-06 or 280. Then go and find some back issues from any magazine and look for the what's better 270 or 30-06, then after that gets old, find the ones that add the 280 into the mix , then go and throw up, disgusted that they would even print that drivvle ,worst yet someone would even spend money and buy it.
Yes you can use brass from 308 for a 260, yet you can also use 7mm Mauser brass for the 257....and you would do that? WHY? SHORT ACTION 257!!!!!!!!!! You Swampman have read to much of the drivvle and I hope you paid ($$$$) dearly for it. Many thing are written for know other reason to sell copy, it is then repeated over and over and over again, by people like you, until everyone is an expert and it is now the fact as opposed to fallacy. The one downfall the 257 "HAD" was Remington "NOT" loading it to its potential, "THAT" is not an issue anymore. BUT, our questioner is a handloader so that is now or better yet, never will be an issue.
Both, will do what you want, not one better than the next. UNLESS, you believe in paper ballistics as unfortunatley most use for an excuse why one is BETTER than the other, while in the game fields, actual real life hunting, a deer hit properly, will not know the difference and neither will you. So, try this, take the bullet of choice in both 6.5mm and .257. Carry them around in your pocket all day, at the end of the day, reach in, pull "ONE" out, and that my friend is the one you cannot go wrong with. RayS.. made a good point, since you do reload, the 6.5X57, now you have the best of both worlds, OH OH!...., now that brings up a new discussion, Should or Should we not, give it one of Parkers 40 degree shoulders? Decisions decisions!!!!!!!!!!!CRASH87