I have both of them, a 457S and a 4516-2.
Thoughts:
The 4516 is certainly a durable, well made and machined all steel 3rd gen Smith. You'll know it's all steel if you carry it any appreciable time. It does not pocket carry all that well, although it can be done. It is also heavy IWB. Good range gun, nice traditional sights, good occasional carry gun but really needs a belt holster to carry it comfortably.
The 457's alloy frame makes it a bit lighter to carry, its sights are not as nice but work well for the purpose, it is not as finely contoured in slide or frame. I find that mine is more accurate than the 4516 with the same loads and actually seems to shoot softer...perhaps it is the double captive recoil spring it has that is different. In fact, I have had ladyfriends shooting them side by side and they preferred the 457 because it is lighter but counterintuitively seems to recoil less to them. I thought this so myself but it was interesting to see it corroborated by at least a few other shooters who are far more recoil sensitive than I am. The triggers are about the same on both.
I wouldn't readily trade or sell either one because they both have a purpose but if I did it would likely be the 4516 because I can use a 4506 for a range or heavy duty carry gun. Otherwise, if I wanted a 45ACP to use with mostly heavier +P loadings and practice with such more frequently, say to use as a field/woods carry option in a belt holster, the 4516 would be my choice. If I wanted more of an urban carry gun using mostly standard loads for practice and +P for carry, then the 457S would get the nod. Not that I think it is necessarily that much less durable ... most folks probably shoot no more than a 1000/1500 rounds a year anyway in their pet carry gun unless they really practice a lot or use it for action games.
You can't really go wrong with either. Since you already have a 3953 and presumably carry it, you will find the extra weight of the 4516 a lot more noticeable if that is a factor. I would like to find a 3953 (have a 3913). I have it's big brother 4053 and actually choose to carry it more often than either of the 45s. Great gun. Wouldn't a 4553 be just the thing?