What is the difference between the viper and the raptor (other than looks)?

Thanks, that's exactly the info I was looking for.

On the Viper: The access to the reservoir is a simple velcro tab to open the pocket. THe backpanel is spacer mesh and foam. The hipbelt is small AND removeable. The framesheet is reduced--HDPE framesheet molded to fit the handle on the reservoir--so it won't carry weight well BUT it is also lighter. On the Raptor: The access to the reservoir is a zippered pocket that squeezes the reservoir a little better than the Viper's pocket. The backpanel is a stretched mesh over a rigid corrugated foam for better ventilation and weight carrying. The hipbelt is thicker with stretch pockets. For the upgrade to the Raptor, you get a slightly heavier pack that breaths better, carries weight better, and stabilizes the sloshy reservoir better.

The Viper (men's) has a "detuned" hipbelt, harness and packpanel, and a slightly different "hydra lock" system on the inside to carry your reservoir. The inner pocketing is the same as the Raptor. The Raptor comes in two sizes, S/M and M/L. The Viper comes in one size only, and fits like the Raptor M/L. Let me know if you have other questions!