What are the pilots around you flying?

If everyone else flies 15m class & you buy standard you'll have to push
really hard to fly with / against them & will land out a lot.

If everybody else flies standard class & you buy 15m class then you won't
be pushing yourself at all, and it's pushing yourself (a little) that will
improve your soaring.

Neither of the above scenarios are good in your early years.
(Or later years for that matter)

And it's in the nature of the sport that you WILL want to fly with /
against the pilots around you.

With any glider from the LS4 to the JS1 the limiting factor is likely to be
you not the aircraft.

Regards

KN