Hard to explain because there isn't a big difference between those two other than what would be the model of scope they are build in. They are both vipers, but the 6.5-20 is the biggest of the "just" vipers (offered in 3-9 to 6.5-20), and the other is part of the HS line of vipers. Both have same glass, turrets and reticles but the small differences are in the eye relief and eye box (you can see the the zoom levers are different), field of view and MOA travel. HSTs, HS-LRs and PSTs again have different features but all under the same Viper line.
I'm currently running the Viper 6.5-20x44 on my .204 and 25-06 and they work great. Compared to the other Vipers I have the eye relief is the most noticeable. The 6.5-20 has the smallest eye box and the least amount of eye relief.