I have owned a Vortex Viper PST 4-16x50mm, and I currently own a Weaver Tactical EMDR 3-15x50mm (it's the "whore" scope that gets passed around from rifle to rifle, loaned out to friends, etc.).
The PST has what I feel to be a better reticle, better-feeling knobs, and the CRS "poor-man's zero stop" system. I also think it has a better illumination knob, providing an "off" position between each level of "on", and the illumination settings are quite appropriate for the scope's intended use.
The Weaver has locking knobs which can be nice if the rifle is getting banged around or constantly pulled from bags or vehicles. It also has remarkable glass for the money; I typically don't give much of a rat's ass about optical quality, but the Weaver is stunningly good. The reticle is a nice improvement to standard the standard mil-dot. The scope construction is quite robust; while I wouldn't use it to drive tent stakes, it feels stouter than the competition (the example I own has not led an easy life, and is holding up fine - including a few rounds on a .50BMG).
If I have a complaint about the Weaver, it's the illumination; while having two colors (red and green) may be useful to some, both of them are much too bright for twilight or nighttime use, even on their lowest settings. The brightest settings are almost bright enough for daytime use, but by that point, much of the reticle detail is getting washed-out.
In short, they both have some strengths and weaknesses. The right choice for you will come down to how you value certain features and aspects of performance.
