1:9 may work "fine" but 1:7 will work better for your typical AR user who uses 55gr and up. Why pigeon hole yourself into light/short loads w/ 1:9? Are you really going to be putting anything lighter than 55 through you firearm? To be honest I highly doubt it from my experience.
You mentioned target practice. 1:7 will prove better w/ the higher grain ammunition which is better for target shooting. There's a reason most of your higher quality rifles come w/ 1:7 barrels.
As far as in between 1:8 is in good between and often used in stainless barrels that tend to be made for greater accuracy. To me thats a big clue.
I understood the following, from a gun shop, regarding AR15 M4s he had for sale:
Bush Master, cheaper but you couldn't shoot .223s only 5.56, no higher grain bullets.
Colt Law Enforcement/Military .223 and 5.56 and higher grain bullets.
I understood they both were the same rate of twist.
Just by playing averages the Colt w/ sling and magazine are probably closer in value to the asking price. What model Colt? You can find 6920's for closer to $1k, add shipping and FFL and you're still saving yourself some money.