if you're looking to build a clone of a real MK18 and want to use fixed sights, go with LMT. If not then the DD ones will probably work better for you (no big knobs).
I run an EOTech on mine. I like it but have gone through but the battery life isn't the greatest. Still think they are the best optic for quickly aquiring a target. Once you use a holographic weapon sight, it's hard to go back to a red dot/Aimpoint style optic. The technology is amazing. It's designed to shoot with both eyes open.
Aimpoint has a reputation for good battery life. I have been interested in a T1 Micro for a while. As a small, lightweight optic, it seems to compliment a short barreled rifle. A quality mount (LaRue, Daniel Defense, etc...) will cost over a hundred dollars more.
Having siad all this, I sold two pistols and will be selling my EOTech XPS3-0 in the EE to fund an Elcan SpecterDr 1-4X. I am building a clone and if you have not seen an Elcan in person it is a great peice of kit. They also look badass on a MK18...