I've owned both, I have had a 950 adventure and I currently own a 950 SM; but I did run an 800GS for a year...
The is simply no comparison, but I will try to be balanced... Er no I can't be, the 950, in either adventure of SM flavour is different league, the 800 doesnt even warrent comparison, it is worse in every respect!
If you want to travel on it go for the adventure, I did 20,000 miles round the world on mine and it survived with only a couple of hiccups... The SM will do the job but you will get a bit more battered due to the lack of screen etc, that said it will be more a laugh because it handles better with the small front wheel, and my god it turns quickly!!!
The finish of the 950 is better, the 800 was really ropey, and started to look is age after about a year, my SM is now on 14,000 miles and still cleans up like new.
Most 950s will now have the common issues sorted out, read through here and get a feel for them... The only thing I have experienced is a failed clutch master cylinder; happened to me in the desert in Iran which was awkward, but it got fixed, I have not had any issues on the SM. Both types are really easy to live with and not uncomfortable, actually both are much more comfortable than the 800. Servicing is not too horrific, I have just had the 13500 mile one done and it cost 150 quid-ish I think, the 9000 / 18000 ones are a bit more cos of the valves but not an issue.
If you get one bin the exhausts as they run hot and are heavy...
I havent really said much about the 800 as there is nothing to say, it does what it does well, but is dull and not as good. Just buy the 950 and don't look back!!!!
PS, just seen you mention 950 / 990... Go for the 950, the carbs make is a much better propostion than the 990 with its injection. I think it is better now, but the 950 is perfect!!