In reality, there isn't much difference between the two types you mentioned. The SATA 6G boards simply have SATA 6G and USB3 controller chips added to the PCIe bus. This means that these boards do not natively support these new technologies (ie controlled by the southbridge), but use extra hardware and leech PCIe bandwidth to make them work. There are other minor changes on the boards, but I certainly wouldn't discount the older boards.
As for which to go for,
this one is a good deal imho. It has the SATA 6G/USB3 features (nice to have but not exactly essential) and it does crossfire (no SLI) @ x8x8 speed.
Edit: on further inspection, that motherboard only does x4 speed on the 2nd PCIe slot - so if you ever plan on running it with a crossfire setup - I wouldn't recommend it. As a single GPU board it would be great.
If you do want to do crossfire, the cheapest P55 board is
this one for £150. However, it is hard to recommend as it is rather expensive for what it is (only x8x8 crossfire) and you can get
an X58 board for the same money, which would run better will a multi-gpu system (x16x16 PCIe slots and supports SLI). Also, if you want crossfire, the AMD AM3 options with x8x8 and x16x16 crossfire support are a good deal cheaper than these options.
