Do you really mean that it's several years before you have to decide? If so, things may well have changed. You may be looking at a very different 1200 capacity bike, if indeed it's a 1200 at all.
If you are looking today:
It depends on what you want, as ever. I have owned both the vanilla and GSA versions and (for a while) both at once.
When I bought the vanilla bike the GSA had not been released. I clocked up 18,000 miles in a year, mostly in Europe and on one trip to Morocco. This was spent 95% on roads with either BT020 tyres or Dunlop Trailmax's, the 'halfway between' tyre. Both types of tyre could cope quite happily with 'off roading' on gravel or broken roads in Morocco and in Spain / France. The tank range is good enough for most things, the bike 'hoons around' as evidenced by the three track days I did on the thing.
I switched to the GSA because of the type of riding I do. I do not ride that much in the UK at all. I live in central London. I have no need to commute on a bike, as I walk to work. If I want to go anywhere in Town I can use public transport, call a taxi or take the car; it is so much easier. Most of my riding is holidaying in Europe, with many days over 300 miles on D or N roads. I like the increased tank range; when my mates stop for fuel I can miss every other fuel stop, near enough. This gives me perhaps 15 minutes to relax whilst they faff about. I like the larger build of the bike, that it is a bit heavier (but no stronger really), the OE luggage is good (but so is the vanilla bike's) and easy to use. I like the bigger screen but never seem to drop it down. After 30,000 miles in two and a half years, I have got used to it and can, if I want to, take it up the same gravel tracks on its hybrid Anakee tyres, without too much drama.
If I was just using a bike for charging about the UK at weekends, a bit of a commute (perhaps) and doing perhaps 5 to 7,500 miles a year, with a European holiday lobbed in now and again, I would go for the vanilla bike. It is cheaper and more than capable.
If I was doing only longish tours and several of them in a year, I would look carefully at the GSA.
Whatever I did, if I had never ridden a BuMW or a 1200, I would certainly ride both a couple of times, making use of dealers' demo versions or mates' bikes, before parting with my cash.
