There is no difference
seo-wise. The AddThis buton is a link just like your custom button.
The only difference is that with addthis you are limited with the design options (button size, colors, etc.). If you think you can get a better "effect" (I'm talking about influencing people to vote for you) using your own buttons, go for it.
Why isn't everyone using addthis? Well, addthis is some sort of NKOTB, plus there are a lot of other options.
LE: I stand corrected
Button customization - You now have full control over the customization of the the button image, text, and style. We essentially moved the button image and link outside of the javascript, so you can now change the image, text, or style of the button as much as you want, and very easily.