I Personally think that cv is the best way too loose weight but to generally tone up weights work well and some say that done right you can loose alot from doing weights alone,if your trying to shed fat in the tummy area do alot of work on an exercise ball or something that keeps your core working (sounds cheesy but its true)

Dont go too heavy with the weights sacrifice going up a dumbell for another couple of reps and keep you for 100%

I used to do sit ups near enough everytime i trained but not alot of cv so never got a blinding 6 pack because was still caryying a bit of weight due to trying to get bigger, none the less i have a really strong stomache so if your trying to loose weight combine crunch's situps russian twists etc into your cv workout i'd say,see if that works if not try another approach

But if its a 6 pack your after you would generally have to drop to a really good body fat %