It would be better for the new owner if you left the SIM card in. They will need a SIM card if they wish to use the 3G.
I suggest leaving it in - but make sure you cancel the account that it's activated with. That way, they can't use your money to use the 3G and will have to set up their own account.
So, unless you need it for some reason. I'd suggest the SIM card go with the iPad1.
Marilyn