Ian,
I havent used an IBC in Scorpion Jr. yet, but I have used both in Pillow Torque. As for which is better, my personal view is there is very little difference in overall quality of control. Although as Alan says, you do have the option of changing the ramps and curves on the Vantec, I have also found the default setting (fast ramps & expo. steer) is about as good as you will ever need.
The IBC is a true linear mix (equal throttle and steer curves and wickedly fast ramps) which is VERY precise. In a 4WD machine such as Pillow Torque the IBC gives very good results, but the linear mix and very fast ramps can make the throttle feel slightly twitchy and the steering feel a little clinical but you do get used to it very quickly. The Vantec on the other hand with its expo. steering curves and slightly slower ramps does have a softer feel. (You can program the Vantec with a true linear mix if you prefer it). The IBC does have the advantage of an onboard BEC to run your RX and a flip function that reverses the throttle input if you machine can run either way up and you also get 2 switched outputs for weapon control. You dont get any of those on the Vantec. I have found that the fail safe software on the IBC is VERY sensitive (no bad thing) so you do need a good radio set up and well suppressed motors or the IBC can fail safe without warning. The Vantec is a little more forgiving of the odd bad radio signal but is still more than good enough to pass any tech check.
Hope this is of some help.

Geoff.