Most people will tell you that neither of those cards are worth spending a penny on. But, if you had to get one, the R7 240 has more raw power. Take a look at this
review of a R7 240. In it, the reviewer points out that a GPU such as the R7 240 is not a good buy because of the poor price to performance ratio you get compared to a card such as the
HD 7750 which in US dollars only costs around $10-$20 more. An R7 240 is around $80-$90 USD and a HD 7750 is $100-$110. The thing is, the HD 7750 gives you almost twice the performance of the R7 240 for only around 15% more in price. Even though the base HD 7750 only has 1GB of VRAM, it is a better type of VRAM (GDDR5 vs DDR3 of the R7 240) and it has more stream processors (512 vs 320). My suggestion is that you save a few more bucks and get at least a 7750 or GTX 650 (128-bit GDDR5). Or, if you can find one on sale, a GTX 650ti, GTX 650ti boost or a R7 260x. Anything under $100 is really not worth buying these days with APUs and integrated graphics providing almost the same amount of power.