Neither are really powerfuly for gaming, if you have to pick between the two the GT240 would be better.
Why don't you tell us your motherboard, cpu, ram, resolution, psu, and budget and maybe we can find you something better for your needs.
motherboard: Gigabyte G41MT-ES2L
CPU: Q8400 core 2 Quad 2.66ghz
Ram: 2GB DDR3 1333Mhz
PSU: Thermaltake 430watt
i will usually want to play fifa, nfs, gta, cod.