Are you looking at the older "5006" series receivers? Or do you actually mean the SR5010 series? Marantz is currently up to the SRx011 series now (i.e. SR5011, SR6011, SR7011).
Generally speaking, all three of these models are pretty much the same as far as firmware and functionality. The upper model may have a couple/few extra inputs or outputs. The main difference is in the size of the power supply, which means better/smoother power for the amplifier circuits. The SR6010 sounds a bit better/smoother than the SR5010 (less harsh). And the SR7010 sounds a bit better then 6010. Buy the best one you can afford.