So, a straight question, what would you get if you were in my shoes, with my avionics set-up/home-proximity to busy glider site ?
I would get the Rosetta.
I genuinely don't have a dog in the fight apart from being a PAW customer and someone who has a mild geekery interest in these things.
If you didn't have ADSB Out through your extended squitter then SE2 would add that very great benefit. But you do, so that is a bit of a wasted SE2 capability. SE2 will see ADSB, so will Rosetta. SE2 is going to cost you a bit more than Rosetta but I get the impression that you're a bit like me and would happily spend a couple of hundred extra if you get the benefit. With this in mind, we'll just ignore the the fact that you'll save a bit with Rosetta.
Without going into the future promised developments of either system (METARs etc), Rosetta, over SE2, is going to give you PAW Out and In and Mode C bearingless In. After that it will be Flarm in direct from SE2 versus Flarm and Mode S multilateration from re-broadcast for Rosetta. Keevil does have an OGN station but it is broken at the moment as they need a new antenna. I reckon even the RAF can sort that one out, so I would be confident that you'll get all the info you need about gliders once they get going again and start broadcasting.
It's a shame Avmap don't do a remote-mounted magnetometer, or else you could plonk it in the wingtip (or similar) but, failing that, I would use the SkyDemon wind vector for the purposes you need. As I said before, you only really need it to determine which end of the strip you're going to land on and SkyDemon's calculated wind (I think) will be good enough to do that. Forget the fact that you don't have a magnetic compass and rejoice at all the other fancy gubbins you've got.....and the weight you've saved
When you've finished installing the kit in your aircraft then talk to Keith at Pilot Aware and set up your own ATOM station at home. Three antennas, a Raspberry Pi and an ethernet cable and you'll not only help others by re-broadcasting in your area but you'll make sure you always see the gliders from your own super-duper Flarm antenna at the house