Will be buying the solo air to go on my Trance. Is the 20mm option far superior or is there no need, therefore will the 15mm do the job? I want it very stiff, so to speak.

Don't bother with 15mm, its just some half way measure that aint worth bothering with IMHO, and the weight saving between 15 and 20mm is negligible.
I've got 20mm maxle lite rebas on my HT and they're superb.
Unless there's a huge price difference between them then 20mm every time.