Since HDDs have trouble reaching 150MB/s; using SATA 6Gbps (600MB/s) won't affect performance much. Also, sequential performance is only relevant if you do basic copy tasks alot. For the system drive, containing applications and the OS, random IOps performance is much more important.
Below screenshot gives a nice idea why SSDs are so much faster than HDDs:
As you can see random IOps performance is much higher than any HDD is capable of. No harddrive can even reach 1MB/s when doing REAL random I/O. So forget those 100MB+ scores you'll only get that when doing simple copy tasks. Its IOps that matters.
Perhaps you should read about what SSDs are and what benefits they have over HDDs, before you proceed any further.