I do believe, when you buy just "mint" at the market... it is spearmint...
When you buy peppermint, it's... well peppermint...
I prefer the taste of peppermint to spearmint myself... I like a few leaves crushed up in iced tea once in a great while...
I also like it chopped and added to a fresh garden salad...
and always lots of it, chopped and crushed, added to my rub for a Leg of Lamb (along with a lot of garlic & rosemary)!