between the two 7850's really the main difference is the display outputs...the second one has two dvi ports hdmi and full sized displayport...the first card has one dvi two mini dp ports and a hdmi....if i were choosing between the two I would get the second card dvi is more popular....also I would get a 7850 over a 480 any day but of all of the cards I would get the 7870 for 5 dollars more without a doubt....go for the 7870 that is a really good deal.
the directcu II is just the type of cooler that asus puts on there...they brand it that its really nothing crazy or necessary...it appears that the 7870 you picked out has that type of cooler too....it doesnt hurt but every brand has their own type of cooler
I believe all the asus 78xx cards use the direct cu II cooler, they just only listed it in the title of one of those 7850s. It's just a marketing name for their cooler, it's better than stock but not signifigantly better than any other manufacturers 2 fan custom cooler. The 7870 is definately worth the extra 5 bucks.