Nikon would be an excellent choice, I would also recommand Leupold VX2 tactcail scope with target knobs. This scope is build with economy in mind for smaller dept whom can't spend a thousand dollars on a precision scope, but the quality of the less expensive scope is still first rate. it is a 3-9x40 with an 1 inch tube. This is an excellent choice at 318 dollars for duplex, and 389 for mil-dot reticle. also available from
www.SWFA.comACOG is great for close in to 300m, the Leupold or Nikon will help you reach out and touch someone. Stay away from Elcon, heavy and not very good at retaining zero.
For a great qucik detach mount and ring combo, i would look into getting the ARMS set up. this would be the #21 ring with #35QD throw lever mount. This combo will give you the ability to quickly detach your scope or attach the scope with out the lost of zero. This is also the set up prefer with the SPR project used by the go fast boys over sea. check with Wes at
www.MSTN.biz for price and availability. I think this setup will be about 150 dollars, this will bring your grand total with a Leupold Tactical 3-9x40 scope to about 470 dollars. by the way, using the ARMS ring, you will need a 30mm adapter, which is another 25 dollars. I figure for around 500 dollars, you will have a very awesome set up. this is about 30% cheaper then buying a ACOG.