Go with the GTX 560 Ti. It's actually designed for heavy lifting (hence the GTX), whereas the GT 630 is not. In fact, the GT 630 is nothing but a rebranded GT 440, whereas the 560 Ti was a replacement for the GTX 460/470 (depending on how you want to see the performance and labeling).
PS. Consider a Radeon HD 7850 as an alternative. They're not terribly expensive, but run much cooler than the 560 Ti cards and have some performance advantages.
[size=85]Case: Define Mini
Parts: P8Z77-M Pro µATX, i5-3570K, STRIX-GTX960-DC2OC-4GD5, 8 GB G.Skill DDR3U, Xonar DX, WD G 1 TB, mx100 256 GB SSD, RX-5300 PSU
Cooling: Noctua NH-U12P SE2 + Scythe SS PWM, 2x Noctua NF-P12
Extras: Eaton UPS, Dell 24" EIPS, Ducky TKL, SteelSeries Sensei Raw, Synology DS213j 6 TB NAS