Both FN 50.00's and H&K 91's are excellent rifles.
The market on H&K's is pretty soft right now, you can find them all over the web. Real FN 50.00's are a little more rare.
The ergonomics of the FAL are superior to the H&K, but then they are superior to most any other rifle. The H&K is still pretty fast handling, and they shoot well. If you get the TAC LATCH installed, mag changes are much quicker and easier. Sights also seem more rugged on the H&K than the FN.
I don't understand the "brutal recoil" comment either. Yes, the FAL is softer with the gas properly adjusted, but I have no problem shooting the H&K's either.
Again, both are excellent designs and both have served in various conflicts all over the globe with honor, but for pure investment value, the FN is the better choice in my opinion.
You'd be very well armed with either though.
Good luck in your decision.
Of course, the proper decision is to buy both!