
Salopettes are warmer, better for climbing as they don't pull down to leave a cold spot with a harness. They also have a high front which means they are warmer than the pant. Arguably better for extreme cold weather use.
Pants, not quite as warm overall due to lack of high front, more versatile all round and easier to put on and take off.
For winter climbing and mountaineering use, I'd definitely go for the salopettes. For more general walking use, the pants are arguably slightly more versatile and you could always use them with clip-on braces if you find they work their way down.
All round, I'm probably more of a salopettes sort of guy, particularly if the price difference is only 10 quid, but it's very much down to personal preference.