The 5.1R is essentially a "boys" version of the Hi-Capa series. Blowback strength is the same, but with lower velocity.
There are just as many aftermarket parts for the 4.3 as the 5.1, because for all intents and purposes, the two platforms are exactly the same and all parts are cross-compatible. It's not hard at all to convert a 4.3 in to a 5.1 and vice versa (I just finished building a 4.3 in to a 5.1 1911 straight barrel).
I find both handle well... the 4.3 looks nice, but I always say this about block cut slide/frame setups like the 4.3: It's not as easy to holster, due to the large/square muzzle profile. It looks nice, though. Not a huge issue, but it's something to be mindful of, especially if you're using a soft holster.
As far as CAPS/IPSC/Action Air type of practical shooting - it is not uncommon for a stage to require you to shoot with your left hand. If the situation forces you to draw the weapon and take it off safety all with your left hand, then you will need to be mindful of what pistol you end up going with. The standard Hi-Capa 4.3 has a right handed safety. The 5.1 comes with ambi safeties, as does the dual stainless 4.3. This is the main reason that I tell people to pick the 5.1 if they're working on a tight budget (the dual stainless is a bit pricier), but ultimately, if you really like the looks of the standard 4.3 and have a bit of money in your budget to change things, it's not hard at all to slap on ambidextrous safety levers.
The integrated rail has no place for practical shooting use, but if you're using it for skirmishing, the integrated rail is nice if you're planning on using a combat weapon light or even better, a LAM. The add-on rail for the 5.1 hangs a bit low and does have a tendency to move over time - so if you're using a laser aiming module, the zero can go off.
