Boston's Gun Bible is a good educational read,but it's out of date.
Much of the info from there is based on a time when quality HK-91
clones were not being produced except by custom builders that built
one rifle at a time.
JLD has changed that situation .
Most people here are going to recommend the FAL,it was succesfully cloned by
US manufacturers well before HKs were.
Both firearms have there plusses and minuses.
After my experiences with the M1A ,a Belgian FAL ,and an original
German built HK-91 ,I decided the HK-91 was the rifle for me.
Here's some info to think about .
PTR -91-Lacks a last shot bolt open device.FAL has it.
This is usually considered the #1 deficiency of the HK rifles.
AK-47s don't have them either,also many countries that used the FAL
had this feature disabled because it can fail.
Standard barrel of the HK/PTR is 18 inches.
The FAL barrel is longer ,I think it's 21 inches.
Longer barrel=higher velocity bullet for less bullet drop .
Shorter barrel =more compact design
FALs come in both Metric and inch pattern receivers and parts.
These parts are often incompatable,as well as the mags.
HK parts are HK parts.
Many will say the FAL is more accurate,but no succesful sniper
variant of the FAL was produced.
HK-91s were later turned into several very succesful sniper designs,
several of which are produced to this day.
SG-1,PSG-1 ,MSG-90 are the ones I know of.
Currently excellant condition mags for HKs can be found for as little as
$1 (one dollar) a piece.
FALs are very dependable,but the delayed blowback system of the HKs
is one of the most dependable ever made.
HKs will eat ammo that chokes FALs or M1As ,in severe conditions.
Inherantly ,HKs become quite dirty when used ,and require more
cleaning to properly clean the internal barrel flutes.
FALs are much easier to clean .
Last on my list ,the one thing that won me on HKs.
FALs loose accuracy quickly after intense rapid fire,as
do all weapons.This is also a weakness of the M1A.
HK series firearms do not loose near the accuracy after rapid fire
because of the fluted barrels.
They cool of much quicker and maintain there accuracy more after
rapid firing.
Both have their good and bad points.
Ergonomics are also a preferance to consider,one that requires
you to handle these weapons for yourself.
I'm completely happy with my PTR-91 .