Re: FNX 45 Tac or H&K CT?
If you have really big hands get the FN one, because if it fits in your hand you get 15 rounds in the mag which is a nice plus.
If you have normal sized hands, get the HK45 (full size, not the HK45c or the USP 45) with the extended threaded barrel and you'll love how the grip feels in your hands and how well it points & shoots. The only down side is 10 round mag capacity.
If you need something more easily concealed then get the HK45c either with the extended threaded barrel, or add that later.
If you get the bug, I'd actually suggest getting both the HK45 and the HK45c, The HK45 for your range gun, bedside gun, battle pistol etc and then the HK45c as your daily concealed carry pistol (the same 10 round magazines will fit both if you use the standard floorplates).
A lot of people suggest just getting the HK45c which is a good idea if you can only have 1 and need to be able to carry it concealed, however the big brother HK45 is a superior gun if you don't need small / concealed, or can have both, it has much better grips & shoots even more smoothly.
HK customer service is actually quite good especially since they opened the USA factory, if there is a problem, call them up, then ship them the gun & they will fix it for you & let you know how much it costs (if any depending on the service) and then ship it back good as new, with a pretty good turn around time.
Depending on the deal you find, if you get one with a standard barrel, you can just order the Extended threaded barrel for either one and have a second barrel.