Stay away from the sportline. They are complete and utter crap.
Get the same gun in the proline. Even has a metal body.
FYI, dont be fooled by the full metal sportline guns either. they are still crap cause they have the shitty sportline internals
dont cheap out. im not saying to invest $2k into your first gun off the bat, but buy something decent that will work and leaves you room to upgrade in the future. the CA Proline M4 RIS is a great buy. You should definitely look at that for your first gun.