If you are considering a high end new or newish high performance glider, the obvious choice is between an ASH 31 and a Ventus 3M (shows my prejudice against DG's and full size open class gliders)!. At significantly lower cost it could be an ASH 26E or Ventus 2 CM. I think I would choose a Ventus 3M

I have had two motorgliders, a Wankel engined ASH 26E and a Solo engined Arcus M. The engine technologies have pros and cons. The Wankel is as others have said perfectly smooth but it needs warming up and cooling down. The Solo engine may be fractionally less smooth but I really don't find an issue with it in practise. I think most of the problems experienced with either type have been from causes which have nothing to do with the internals of the engine - things like pylon position sensors out of position or going wrong, exhaust issues, or aspects of the electrics. Most of the time either type will do a good job of self-launching or self-retrieving, and an adequate though not great job of repositioning.

In considering the advantages of an ASH 31 over an ASH 26, apart from the availability of 21m tips as well as 18m tips, consider that 26's are getting to an age where they are likely to be less reliable (the one I used to own currently has wiring issues, not for the first time) and may need refinishing now or before long. The 31 also has injection whereas the 26 has a somewhat mickey mouse carburettor, which should make the 31 more suited to high altitude flight as well as more reliable.

Currently, I would not be ready to buy an electric self-launcher - I would want more endurance if buying a self-launcher. My calculations suggested that the petrol Arcus has about 8 times the endurance of the electric version (of which extremely few have been built) and I heard of someone switching off the motor after climbing to 500 feet in order to preserve battery for a potential self-retrieve. Admittedly that was for flight in very hostile terrain, but I like to think I have enough fuel on board to climb to a reasonable height, to relight if necessary, and to self-retrieve at the end of the day. I must admit that I have never needed to do all that! But I have certainly enjoyed being able to taxi back after tailing to stay up and immediately take a relight without worrying about the endurance I have left. I expect that as the energy density of batteries improves electric engines will become dominant, and I worry that this will affect the sale-ability of petrol engined self-launchers.