The 4.3 is shorter, and has full length rail built into the frame.
The 5.1 comes with separate rail that can be attached by opening two holes in the frame with an implement provided. This spoils the appearance of the gun somewhat as the rail does not sit flush.
Some people think the 4.3 light bearing gun is better for cqb because of its shorter profile and ability to take a light immediately.
I prefer the 5.1's longer appearance, and run it without a light.
There are more custom parts available for the 5.1 than the 4.3, if that is important to you.
Performance is probably about the same for both guns ie. excellent