private sale has the best opportunity for maximizing what you get for it, but a lot of folks in that price range want to go through a dealer, so you could set a time frame like two months trying to sell and see how much interest it gets...but if you're already eying another vehicle you're probably wanting to move quick so you'll have to go get hosed at a dealer.
small - in what way? if it's seating and yo'ure talking a legacy sedan, then i consider it a small upgrade, but i'm not that picky. if it's utilitarian/cargo space and you're looking at a wagon, then it's a huge upgrade and worth it if you need it. if you travel, hobbies, gear, stuff, kids, pets...the wagons are fantastic.
There is one large mechanical benefit: 2010 Legacy gets the updated MLS headgasket so there should be far less chance of headgasket failure in that than the impreza (a $1,500+ repair at Subaru).
But spending $5,000 more to save $1,500 doesn't make financial sense so not a big deal really, but put that on your PRO'S list if you're really itching for some excuses to buy.
$14k does sound really cheap for a 2010. they're usually like $3k - $5k more than that here with 30,000 miles.
if you have any inclination to hold the impreza, get two more years out of it and look to get into the better (hopefully) FB engine, which i think started in the 2012 legacys. no timing belt maintenance or headgasket issues. though they are new and not entirely "proven" yet either. but i'm weird i buy stuff and want 300,000 reliable and easy miles out of it, so i'm picky.