according to notebookcheck's graphics card benchmark list, the 520m is about the same as a radeon hd 6470m which is a card that i have repeatedly been told is only marginally better than the hd3000 and not worth any extra cost.
On the other hand, the i5-2430m is the newer version of the i5-2410m and is really only 100mhz faster... you won't even notice.
I would say go with the gt520m if you have a specific game in mind that you want to play, and that the hd 3000 cant handle but the gt520m can. Otherwise, if you have no specific game in mind then I would go with the i5-2430 just because its newer in general (the whole laptop in addition to the processor).
I would like to say that after reading a lot about laptops, the general feel I get on the whole is that acer is a pretty low-quality, cheap feeling laptop that no one should ever buy. The timelinex series from acer has better quality reputation but then there is a lot of talk about how it over heats and throttles itself. There is a 'fix' for that though. Finally, a personal anecdote about my first laptop which was an acer: i dont know if it was due to the poor quality or my inexperience with taking care of a laptop but it was a pretty bad experience overall with the fan being extremely loud early on (it was mis-aligned, kept clicking against something inside) and then having my graphics card die due to overheating I guess. Also, the lid felt really flimsy but other than that, it lived through 4 years of heavy usage and wasn't too bad on the whole. Of course, none of this is really relevant, but I guess it wouldn't hurt if I posted it.