It would help to know what you're paying for each one. If the difference is insignificant, go for the newer card always.
The 730 and 630 2gb both have similar performance, but the 630 was released 2012'ish, while the 730 came about 2 years later. As a result they both have different architecture, and operate differently to achieve similar results.
GT 730 uses Keplar and overall has much better driver support than the GT630 from their previous architecture. This may be important for some titles that will just not run well on GT630, something to look up.
The 730 is 28nm and the 630 is 40nm chip = the 630 uses almost 3x as much power as the 730. I think it's around 25 Watts vs 65-70Watts. Other thing to note, both these cards run best when used on resolutions lower than 1080p