A infinite wing produces induced drag which is the toll for producing lift. The additional increment of induced drag due to a finite span (tip vortices) varies inversely with aspect ratio so that for A.R. greater than 10 is a pretty small percent of total induced drag. This increment can be affected a little by tip design but reducing an already small number by a small amount doesn't really gain much. This small amount may be worth pursuing if chasing a world record but simply increasing the aspect ratio, even with simple tips will probably give a better return on the effort.
Actually, an infinite wing produces no induced drag. Induced drag is due to vortex rollup, and an infinite wing has no vortex rollup. It does have, however, both pressure (form) drag and skin friction drag (collectively a.k.a. parasite drag).
Using an engineered tip is good if you are limited by span, but not necessarily by tip design.
By the way... I know nothing of sailplane design... so you can roll with it LouW!
