Other than the Experience, use the shortest length.
As for Why you may need to run the longer one my rule (and I think it's a general rule) is that
for 28.6 (small band) use the shortest, only try the longer one if you have a problem with it or if you are running the large band 34.9mm diameter.
Not sure what the experience band is but it's probably the larger, possibly the middle one.
For the reason's why, look at this document and the stroke range, you'll see it's different as the mech gets pushed further from the centreline. that's why the longer bottom bracket pushes it further from the centreline as well.http://www.retrobike.co.uk/gallery2/v/M ... ewsIndex=3
If the bottom bracket on the experience is shimano , there will be MM107 or LL113 stamped on the axle and should be visible if you look for it. that's it's length.
If it's 107, then ace as you can use it to try the raceface/m900 on the bikes.