First off, let me be the first to say..."Get both"!
But seriosly folks, when considering these type of questions I always try to think of which is the harder to replace. Neither model is exactly falling off of trees these days, but both can be found if you look long and hard enough. If it has to be one or the other only you can decide which you want more, or like more. What you need to consider is if you trade the 4516 for the 3914 and decide you miss the 4516, how easy or hard would it be for you to get another one?
Another thing to consider is if you do it and miss the 4516, there are other, easier to find 45's that may fill the void left by the 4516 like a 4566 or 4506. Likewise, if you keep the 4516, you may be happy with a 3913 rather than the -14, which are easier to come by.
I'm no expert but my guess would be that Stu454 is right about availability - the 3914 may be the tougher one to get your hands on. If it was me personally, I prefer the single-stack 9's and I'd do the deal.