The price difference is about ~$36 or a bit more than 55% greater. It's a 40% difference to the X2 550.
Athlon II X2 250 $63 / Athlon II X2 255 $68 / Phenom II X2 550 $88 / Phenom II X2 555 $99
Performance comparison of 3.1Ghz models Athlon II X2 250 vs Phenom II X2 550
Just on price vs performance alone the Athlon II X2 250 is a better value.
Still, the Phenom II X2 550 is only $25 more.
If you're interested in overclocking and you have a decent motherboard that is OC friendly I'd suggest the Athlon II X2 250 and a decent $25 CPU cooler. Sunbeam Core Contact Freezer $25 a/r and free shipping