The 5.7 round can only go through body armor if it is the right ammo (not available to civilians) and is fired from the P90 rifle. The velocity from the pistol is insufficient, even if you could get the right ammo. Where did you get the idea the US Secret Service carries this as their only weapon? Most carry the Sig Sauer P229 in 357 Sig.

The Desert Eagle plays to those who have the idea that bigger is better. If a 45 is good, 50 is better. It is a cannon and brutal on the hand. Recoil is stiff and getting a second shot off quickly and accurately is pretty hard to do. Plus it is like carrying a brick in your pocket.

Of the two, I would go with the FN only due to its more manageable size and ability to fire follow on rounds more accurately. I would not go believe it will go thru a vest.

(However, I think the Springfield Armory XD45 Compact is a better choice. The 45 ACP is a proven round and even if it can not go thru a vest, it is definitely going to knock the guy down.)