They're exactly the same...except for:
4.3
- shorter slide/barrel/inner length (duh)
- single side thumb safety...and it's smaller than 5.1
- lower profile mag release
- fixed rear sight (Novak style)
- commander hammer and beaver tail safety

5.1
- adjustable sight
- dual thumb safeties (large)

5.1 is more accurate (noticeable)...it's controls are "target" oriented. It excels as a field gun.

4.3 is combat. Tight, reliable. 'nuf said.

Care to elaborate on this because this means nothing to me :) ( I am no gun connaisseur).

Field gun? Combat?

Field gun - the pistol I take when I go to a game outdoors. accurate, reliable, holds lots of gas (so I don't need to regas the mags).

Combat - the controls (safties, sights, length, etc...) are setup for low profile, short range combat. I just works.

The only reasons I don't use the 4.3 as much is the non-adjustable sights and that it won't fit my Serpa holster.

Marui in my experience with their GBB is a great platform gun for future upgrades, reliability is great out of the box but they tend to need a lot of care and tuning once you start upgrading them. If you want a reliable field gun that you can bashed around my vote would have to be the newer KSC Glocks especially their G19, i have 2 that I used as my sidearm and they are great they already come with a metal slide and apparently the newer KSC have a more robust internals than the older ones. In comparing my G19 one which was purchased three years ago to the one a few months ago is like day and night, all the weak points have been replaced with a stronger version and that include all the springs, nozzle and Blowback chamber.
ultimately it's your money so it's your choice marui as a stock GBB and a platform for future upgrades are great but there are other options available.

how do you comment on the "new G19" hopup rubber? do they wear down fast?

the new one that i got got a really stiff hop up rubber in fact it's so stiff I have to change it to one of my spares for the older G19, when you compared the two indentical model from the same company it's like oil and water, the same can be said with the Marui I have a few mags from an older Marui hicapa (when they first came out) and a newly purchased one and the weight itself is heavier by 8 grms i know it's not much and the stock plastic slide seem to be a bit more robust than the older model.

I'm currently running a Chrome TM desert eagle and personally i love it, big reliable, rather large mag capacity, and barrel length. the hop up could not be easier to get to, slide the little cover along the top where the ejector is and viola there it is, only thing is that little or no maintenance has a tendency to destroy them :( all in all TM is a great company

any holster designed for a hicapacity 1911 .45 or a 2011 will fit easely. Lookup holster for Paraordonnance or Infinity double stack .45 and you'll be set. I've used teh same holster for my 5.1 and my 4.3....

I used a guarder adjustable holster for all my guns, its holds anything i put in there perfectly

I can custom make silver slides for you in ANY length you want, from 3.4" all the way up to beastly 7" monsters...

... They can go even longer, but functionality and practicality becomes an issue...