They have different characters, some preferring the older air heads for their gentler motors, others preferring the punch from the LC motor. I've owned both (albeit my last one was a much older air cooled one and not the 1200) and I much prefer the LC.
If you intend to tour, you don't really need the extra oomph of the LC but it's always welcome and makes the newer GS bikes the all rounder's they've become known for. There's not really a huge difference in cost between a really good up together TC GS, with lowish miles for the year, and an average LC with average to high miles. You have to do your homework on what to look out for as all have their problems. By and large, the best all rounder I think is the newer 1200GS in TE trim. The GSA adds a massive 30 litre tank, longer travel suspension, higher seat height, crash-bars and go-anywhere looks. It is massive by most bike standards but rides almost as well as the slimmer GS. If you want one to tour on, the GSA is the one most hanker after for the extra mileage that tank gives and slightly less skittish front end under throttle. For more a sporty all rounder the standard GS LC with cast wheels (less bother than spoked) provides a stonking good bike and vfm. All imho of course as others will have their favourites.
Depends on your budget too. By and large, £5-8K might buy you a nice example of just about any GS from the last of the twin cams (probably with plenty of extras) up to a 3 year old LC.
After putting up the first 500 miles on my LC, would I go back to an air cooled model? I don't miss mine and probably wouldn't go back to one but that's not to say it's a bad bike. It isn't. It just feels more under-powered by modern standards.
