The biggest reason I would recommend getting the 5.1 over 4.3 for practical shooting, is mainly because the controls on the 5.1 are ambidextrous.
The thumb safety levers on the 4.3 are on the left side only, so that only right handed users can effectively use it (quickly). Left handed users can access it as well, although it may be difficult for forced left handed shots that can sometimes occur in a stage.
On that note, it IS possible to purchase ambidextrous levers to install on the 4.3, but it's just an added expense.
In my opinion, the extra 0.8" of barrel length alone in the 5.1 will not make the weapon that much more accurate, but when you combine it with the fact that the front & rear open sights are further apart as well as a rear "match type" Bomar type rear sight that is both windage & elevation adjustable, it really does make it a more precise tool for practical shooting.