Both of them will take alot of use; that really isn't a serious concern. I don't mean it's not a good question to ask, but it should not be the factor for the decision. .357 will of course be harder on a gun than .44 Special, but you also have a lifetime warranty - if the gun goes out of spec, send it back.
I have seen some really good deals on the 21-4's now; apparently they weren't strong sellers so the prices are really down. I think they came out at about $900 retail and are now showing up as low as $500 for a blue one and $575 for a nickel one I saw recently (NIB from the dealer, not used).
They are going to be about the same size and handle similarly. The slightly smaller L frame and 3" tube on the 686 will not make a lot of practical difference if carrying is a concern - the L frames are still solid, heavy guns and don't disappear into a pocket or waistband like the K frames can. The thinner barrel of the 21-4 is going to offset it's added length in the categories of weight and concealability vs. the shorter 686.
Your real issue is whether you want a .44 special or a .357, fixed sights or adjustable.
