My reasons would be by just looking at the title, I notice one thing. The GTS 250 has around 1GB of DDR3 memory, while the HD 5770 has 1GB of GDDR5 memory, which is more advanced.
Also, by looking at the specs of each cards, which should be the decision factor, the GTS 250 has 128 stream processing units, while the HD 5770 has 800 stream processing units. Although the memory speed is higher for the GTS 250 running at 2000MHz from the HD 5770 running at 1200MHz. The GDDR5 outweighs the DDR3 on the GTS 250.
The HD 5770 also supports DirectX 11, while the GTS 250 supports DirectX 10. This card is good now, and for the future! It is a little more money (only by around $15), but it definitely better and a great buy. You'll be extremely impressed when you see those graphics flying across your screen.