The VX-6 glass is better than the Viper PST, but it's also more expensive. The dials are what I'm not impressed with as they are a little mushy but it's not designed to be a tactical scope and works better once the CDS is installed. Not sure what the other guy was looking at when he said the PST had better glass but I don't think that is the case putting them both on 16x in low light. Ask an optics retailer who sells both what his opinion is.
I was going back & forth with the Zeiss HD5 3-15x or the VX-6 but the HD5 was a bit limited on elevation per turn and virtually none with stops in.
The PST does have FFP that both the HD5 & VX-6 don't have as options. Personally I'm not a huge fan of 2nd focal plane scopes that have a ballistic hold over reticle as it changes with the power setting so I generally dial and prefer a crosshair with a SFP, but in a FFP like a ranging reticle like the Vortex EBR-1.
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NF 3.5-15 or VX6 3-18? - PredatorMasters Forums