You are mistake as the 625 has about 95oz torque and the 645 has 133oz torque. 625 in speed is .15 and the 645 is .20 in speed which it is a we bit faster but much lower in torque. To see check out the links below.
http://www2.towerhobbies.com/cgi-bin...&I=LXUZ87&P=ML
http://www2.towerhobbies.com/cgi-bin...&I=LXUZ89&P=ML

May be I should not be so hard on Hitec as the servos that went bad where no wear near new but they where both in the same truck and when out close to each other.