I have a small Ventrac 3000, I also looked at Steiner and Power Trac. The PT422s that are owned by another TBN member have been out to my property twice are nice units, but they won't climb my slopes and power a mower deck at full speed. Steiner's were very nice but I liked the small size of the Ventrac for what I needed to do it just was the right machine. I think the Ventracs are a bit more refined than the Steiner machines in most ways. Seems to me that JackIL covered everything very well so I can't add much. But what I do see is that Steiner seems to have stopped or at least slowed their design evolution while Ventrac seems to have taken the designs to the next logical progression in terms of improvements.
As far as tractoring duties like FEL work go, I think Power Trac is probably a better machine. But for slopes, for mowing, and for some other tasks that the Ventracs are well suited to, they are amazing machines and basically unbeatable.