Upload & Sell: Off
55-250, lighter, plastic, non-usm, cheaper, IS and AF like the kit lens, EF-S mount (cannot be used on film or full frame bodies)...$200 new...is a nice starter lens; with the 18-55 you've got a nice kit.
70-300, heavier, mostly plastic, usm for quicker/near silent AF, IS a bit older but usable, will work on all Canon AF bodies...you loose the focal lengths between 55 and 70mm (no biggie in my book) and gain an extra 50mm in reach....$400 or higher new...a better lens all around IMO.
Resale value might be better with the 70-300...
Good luck, either would be great just starting out, have lots of fun above all