Personally I'd invest the cash in the better board. If you plan to keep this for a while and will upgrade the CPU.
For the sake of £8 I'd go with the crosshair, you wont miss that extra 200mhz the 1800x can give you (assuming you overclock the 1700 to at least 3.8) but you may use the extra features this board has down the line.
Thanks. I do believe you're right. I don't usually skimp on the motherboard, so it's probably best not to start now.