Depends what you're doing a lot of on your computer.. if you're playing a lot of games and could utilise the buttons on the mouse then i'd go for the G502 without a doubt. But if you're a general user that plays games from time to time i'd probably say go for this.
i got the G403 from maplin for £24 & its an amazing mouse now looking for the G240 mouse mat for it at a good price
For gaming id go for the G502, you get a lot more mouse for your money and i prefer using it. However ive found the 402 more comfy for extented use, looking at them, i think the 502 is spreading my button fingers to far apart.
Been using the 502 for about 9 months for fps games. It's good but can't really get to grips with the multitude of settings you can apply concerning dropping sensitivity for sniping etc. Nice to calibrate to whatever surface you are using it on and stuff but not seen much improvement over the mouse I paid 8 quid for if I'm honest. This mouse might be a better as a starter.
I'm currently using this mouse, have been for about a year now, and I have to say it's served me well. No real complaints apart from it being slightly smaller than I would like. It'll never come close the classic MX518, but it's good value at this price.