For my 18" .223 rifle, I went with the Nightforce 2.5-10X42 for its build quality, optical quality, size, and features. I put it in a JP mount due to it's rugged yet lightweight design. I use this rifle for hunting mostly, shooting steel, and the occasional competition.
SPR by
wareagle700, on Flickr
On my .308, I went with a Nightforce 3.5-15X50 in a Nightforce Unimount for the same reasons as above. I wanted a little more magnification for as this rifle as it gets shot out to longer ranges than the .223.
MATEN by
wareagle700, on Flickr