I highly recommend selling it private seller. But if you decide to trade it in, have your paperwork saying the car is good to go (compression/leakdown, receipts, most importantly a receipt for the tune and the current mods) and make sure it's in good condition as far as all the basics (CD player, no major body damage, matching tires, no check engine lights, no super flamboyant mods).
If you do decide to trade it in I would go through Subaru Superstore. I know we paid a little more there than we might have paid elsewhere but to be honest the buying experience was excellent. There was no pressure to buy, they gave us a fair deal on the trade and didn't come at us with some insultingly low number, and when we picked up the car the dealer took his time and test drove it with me and communicated with me even on his day off about the financing. Also just the friendly atmosphere and their patience really kept us happy about them, it was the best dealership we've been to by far. PM me if you want to know the guy I was dealing with.