The upgrade to SATA III from SATA II will not give you a noticeable speed boost under normal usage scenarios.
You would only really notice the difference if you are running benchmarks, often transfer large files between a pair of SSDs or run data-heavy professional apps.
Save your money, it is defiantly not worth purchasing another motherboard only for SATA III support.
As another thought, why do you not just get a SATA III add-on card?
They are quite inexpensive and will allow you to run your SSD at native SATA III speeds.
Definitely something I would agree to.