The differences:
REV 3.0 – NEC USB 3.0 controllers ; IMO better
REV 3.1 – ASMedia USB 3.0 controllers ; Cheaper USB 3.0 chipset & CRAZY to charge/spend more $!!!
I'm not a fan of using an x4 PCIe for Crossfire, nVidia requires x8 minimum/PCIe and AMD/ATI opted for a an 'anything goes' x4/x8/x16. The HD 6850 is a decent mid-level GPU so IMO should be on an x8/x8 arrangement.
IMO I'd look at either the MSI P67A-GD55 (B3) or P67A-GD65 (B3) {eSATA + Marvell SE9128},or ASRock P67 EXTREME4 (B3). The MSI and ASRock in your price range makes the most sense.
Side-by-Side ->
http://www.newegg.com/Product/Productcompare.aspx?Submit=ENE&N=100007627%2050001312%20600093976&IsNodeId=1&bop=And&CompareItemList=280|13-130-574^13-130-574-TS%2C13-157-229^13-157-229-TS%2C13-128-478^13-128-478-TS%2C13-130-582^13-130-582-TS%2C13-130-573^13-130-573-TS
General overview of
ASUS P67:
Maximus IV Extreme {2~3WAY, x8...x8, Marvell 9182 [x2 PCIe lanes] RAID 0/1, eSATA, BT, Dual Intel LAN}
P8P67 WS Revolution {2~3WAY & 4-WAY Quadro only, x8...x8, Marvell 9128 RAID 0/1, Dual Intel LAN}
P8P67 DELUXE {2-WAY, x8/x8 + x4 open, Marvell 9128 RAID 0/1, eSATA, BT, Dual Intel/Realtek LAN, USB3 Ft Panel}
P8P67 EVO {2-WAY, x8/x8 + x4 open, Marvell 9120 no RAID, eSATA, BT, Dual Intel/Realtek LAN, BT}
P8P67 PRO {2-WAY, x8/x8 + x4 open, Marvell 9120 no RAID, eSATA, BT}
SABERTOOTH P67 {2-WAY, x8/x8, Marvell 9120 no RAID, eSATA}
P8P67-M PRO {basic} {2-WAY, x8/x8 + x4 covered, eSATA}