I want the extreme 4 for myself but it never goes on sale.
It was on sale for the pre-black friday stuff at NCIX.ca (west coast Canada, but ship anywhere - at the time, $5 shipping anywhere in Canada) $140 - $20 mail-in... I know this doesn't help you, and I also don't know what you consider "on sale".
I've bought 2 Z77 Ext4s from them so far, both worked with no problems other than the ones that come "stock" with the board...
Regarding the thread question, if you plan on 4.5 to 4.8 GHz, probably go with the ASUS... you should get better voltages with that board, and like mentioned, way better fan control.
If you have a decent water cooler (or plan on getting one) then it really doesn't matter which board you go with... go with the one that has the features you want, ASUS has better BIOS/OC features otherwise they are basically the same for features.
I went with the Ext4, it was both cheaper, and looked better... not for any "good" reasons...lol
it is not listed so does that mean its not compatible?
I wouldn't worry about that too much, I know the Corsair memory works, not entirely sure about the Crucial... pretty much any 2xX DDR3 will work, it's usually only with 4xX you start running into problems... especially if you want to OC your memory.
I've got
Patriot Viper 3 1600 2x4... it's not listed on Ext4 compatibility either... works fine, and will OC to 2200 (might go higher, other than quick 2400 testing, I haven't tried)... I run it 24/7 with these settings: 1800, 9/9/9/24/1 (stock @ 1600 but 1N instead of 2N) and undervolt to 1.450v (1.5v stock, I can go as low as 1.4v @ 1800)