This thread was over a year old, so I'm going to close it, but will answer your question.
The M500 is a wired mouse, and the M510 is wireless. Many gamers will tell you that wireless mice can introduce some latency into clicking and moving, which gamers (especially competitive ones) tend to hate with a passion. They may also be subject to interference. Although I think good wireless mice are a lot better for games than they used to be, 1) I'm not a very twitchy gamer myself, so my opinion may be of little value, and 2) a budget wireless mouse probably won't qualify as a "good" one by that definition.
If you have more questions about mice, please open a new thread. Thanks.