The 550 Ti and 250 are more or less the same in performance in most cases, and neither is a worthy upgrade from what you have now. Don't get hung up on model numbers. Just because 550 is newer and the number is high doesn't mean it's faster than an older card. The 550 Ti was actually on the lower end of the 5xx series whereas the 480 was at the top of its series.
If money is tight look for something like a 650 Ti or 660, aim for a 2gb card if 1080p gaming is the goal. Keep an eye on power demands, or you might get a card you can't power. On the AMD side, something like an HD 7850 should be your minimum, the R7 250 is equivalent to an HD 7750, which is a couple of levels below that.
edit: a 750 Ti would be great, just make sure it's a real 750 Ti, there have been a lot of fakes being sold. Always check the specs with the ones on Nvidia's page.