5.1/4.3 refers to the length (inches) of the outer barrel...inners on both are slightly shorter.
Short answer: Get the 5.1
5.1 advantages over 4.1
- ambi thumb safeties...good for when the stage calls for weak hand manipulation
- longer inner = more accuracy (yes, noticeable), more velocity
- longer slide = longer sight line = more precise aiming, better balance
- target rear sight...adjustable (the 4.3 sight is non-adjustable)
- lots more parts are designed for the 5.1...but many will work on the 4.3
- mag release sits a bit higher...the 4.3 is "low-profile...5.1 is better for fast mag changes
* biggest one for me is the adjustable rear sight...I like to aim at what I hit, not off to the side or high or low...