The 560 SE is a slightly-crippled 560 (288 cores vs 336, compared to the 192 for a 550 Ti). It runs at a somewhat lower clock than the 550 Ti. The 560 SE is definitely the more powerful card, and if it's cheaper, there's no question that it's the better buy. You should be able to get some OC headroom out of it too, since it's running at a relatively tame 776 MHz.
The thing I don't know is how the 560 SE compares to, say, a 6850. I'd imagine that it's similar performance, but I haven't found an actual comparison.