You can't go wrong with either, in my opinion. Both are very reliable and accurate. I find the HK to be more ergonomic. If magazine capacity is important look no further than the FN (15 vs. 10 rounds). The slide on the FN is cut and threaded for a MRDS.
Biggest difference is capacity. HK45 (vanilla or tac) are 10 rounds; FNX45 (vanilla or tac) are 15 rounds. Personally I think HK45s fit my grip better than the rather large grip of the FNX45. But then again you are losing 50% of firepower for that improvement on ergo. Also I have not notice any substantial increase in accuracy with the better ergo, maybe just a slight better usability and bit quicker in terms of weapon operation. There are other minor differences like the FNX45 tactical comes with suppressor height sights and milled slot for advance optics while HK45 tactical doesn't. HK does enjoy better finish and machining, but again those don't necessarily translate into better accuracy, none that I've noticed anyways. Both triggers are equally m'eh.
I guess I could have figured out the capacity by looking at it a little closer. I do own a FN TAC and like it a lot. I was thinking of the HK for my next handgun and noticed they looked a lot alike. Is it worth the 1100 bucks?
I own both and I can say they are two different guns
pros - more economic, better fit and finish (at least the polymer parts) , ability to swap to a LEM trigger (IMO)
cons - less capacity 10rds, cost more for the 'tac' model , do not get suppressor sights (WHY HK WHY), expensive german mags
pros - can mount a RMR from the factory, suppressor sights, 15rd of all goodness, its a FN, cheap- er mags
cons - kind of a mushy trigger, BIG gun, weird trigger reset-ness (you know what I mean if you have one) , sloppy polymer molding, I had a faulty mag release
I have both. Bought the FNP .45T first & put an Elite Iron suppressor on it. Works like a champ. Then I decided to get an HK based on forum comments, so I called HK in Georgia & they walked me through all the different models. I wanted a tactical so I could use the same suppressor on it. Decided on the HK .45T instead of the USP Tactical, bought it, had the suppressor modified(reverse threads for the HK), mounted the suppressor to shoot some rounds & discovered I couldn't see the front sight over the suppressor. Guess the guy I talked to forgot to tell me that little fact. Turns out the USP Tactical has a raised front sight, the HK does not. An after market sight is the only answer. If you want to shoot suppressed get the USP version.
I had both but for some reason the finger groove grip on the full size HK just did not suit me. I found myself always having to adjust my grip and didnt like that. I think I am in the minority on that as most love them, just didnt fit me. I really like the FN and the compact HK like Full Assault has is awesome to me as it doesn't have the spiderman grip.