When both of them are in a desert.
-War Gaara's basic sand control is like this. [You must be registered for see links]
-Jin Gaara's basic manipulation is like this. [You must be registered for see links]
That should be more than enough to decide who is stronger than who.
I used to think this too. But War Arc Gaara has displayed some pretty good feats too.
-He can make bijuu-sized sand arms in a location that's not a desertYou must be registered for see links
War Gaara was throwing up Sand walls large enough to protect Suna from C3, so doing that is child's play to him.-He can make massive sand tsunamis even without ShukakuYou must be registered for see links
Keep in mind Jin Gaara gets almost no sleep so he's probably always exhausted and his stamina won't be that great. And he is also constantly fighting to supress Shukaku and keep it under check, since he has a bad relationship with his bijuu. I've seen some good arguments for War Arc Gaara being overall stronger than Early Shippuden Gaara.