It probably won't make much difference. Sony is known for quality products, but the Sony battery is probably just a re-branded Panasonic or Sanyo. I know that Sanyo makes good batteries for R/C car racing, but this is a whole different feild. My guess is that both are about the same.
Rosky is correct: the battery warranty is just one year, and can't be extended. BTW, I've not heard about any reputable vendors who would offer more, and smaller vendors don't mention battery warrenties (in practice they tend to cover batteries for six months only).