Holding off why?
There isnt any benefit unless you are using multiple SSD drives, and even then, you are looking at mega high end stuff.
Doubtful that mechanical drives will even reach SATA-II speeds never mind over that.
Or is it just the motherboard that you want SATA 6Gb/s?
Anyway, there isnt such a thing as SATA 3. Its referred to as SATA 6Gb/s. Due to the confusion of SATA-II, SATA 2 etc.
[/pedantic mode]