Ive got a vx-f 2-7x33, and had a Fullfield e1 4.5-14x42. The only sig ive spent enough time behind to have an opinion on is the BDX, and its got enough features, i think they deduct from cost by reducing glass quality.
I was very happy with my ff e1, and im mostly happy with the vx-f. Both have covered turrets, and track properly. Neither has great turret feel, and id not choose them for spinning up elevation.
personally id go with which ever one is cheaper on that particular day, i really dont thin either has a real edge in optical quality.
Ive also owned 2 ff2s, and while i cant honestly say i prefer them to the leupy, or ff e1, they are about 100-120 bucks used on ebay and are AT LEAST as good as the others optically. The turrets can be kinda annoying, but again dailling isnt what these things are designed for.