I have been making a similar choice (between the Extreme4 and the Asus V LK). You can read some of the info ppl have been giving me here: http://www.tomshardware.com/forum/312062-30-asrock-extreme4-asus-p8z77
The Asrock should support that memory, and it can do a proper crossfire at PCIe 3.0 x8/x8 speeds, which is equivalent to PCIe 2.0 x16/x16.
The Asus LX motherboard has only one PCIe 3.0 x16 slot, and one PCIe 2.0 x16 slot that runs at x4 speeds. It is capable of Crossfire but will run the card in the 2.0 slot at a reduced speed. x8 speed for PCIe 2.0 is usually what is considered to be full-speed for modern GPUs. It is also not compatible with SLI, at least according to the review at Hardwaresecrets, so you couldn't run two NVIDIA graphics cards, only AMD. It uses an older audio chip compared to the rest of the ASUS and Asrock lineup, and only has a 4+1+1 phase power delivery system compared to the Asrock 8+4, so it may limit your overclocking potential.
I think the Asrock is the better choice. It's close to the top-end of Asrock's Z77 offering, and the LX is the very bottom of the ASUS lineup.