i dont know the difference but i do know the physcial limitation is the metal not the electron. Whatever interface its on, the hard drive can only spin so fast and data can only be read so fast so the bottleneck right now is the spinning platters. A disk that spins at 7200 will be faster than a disk at 5400 and a disk at 10,000 will be faster than a disk at 7200rpm.
As for interface, i dont even think the IDE channel has been maxed out yet let alone SATA 150. So in my opinion SATA 150 and SATA 300 ought to have the same performance. However, if you are buying new, get the newer interface, its the same price and i not 100% positive i am right.