It comes down to features.
The 2810 has more capabilities - such as sFlow, source port filtering, and enhanced security capabilities. I think the 2810 also has optional redundant power capability through the 600 RPS/EPS.
The 2810 also does up to 9,220-byte Jumbos, whereas the 2510G falls 4 bytes shorter at 9,216 !
HP ProCurve Switch 2510 Series
HP ProCurve Switch 2810 Series
Andy.