The benefit of the widening kit is that you can choose widths of 4 1/2" and 6'" wide. This can be achieved by moving the A arms and using a different tie rod extensions.This may help if riding wooded trails. Also if you use the lowering and widening option you can raise or lower according to your needs.
The benefit of wider A arms, is a better looking product, possibly more strength, and the added advantage of getting the bike one or more inches longer.
I run the Diamond J widening and lowering kit and am more than happy with it. I can pick the bike up for MX and can drop it way down for flat track and drags.