OK, as promised...
For the purposes of being clear: I have the Leupold 6-20 FFP ER/T model (MRAD). Quality of optics and should be the same as your model. Mine does not have illumination, so I will not comment on that aspect. I have the Vortex Viper PST 6-24 FFP (MOA).
I've had the Vortex for about 2 years (just after they were released? maybe longer?) and I've had the Leupold for a tad over a year and a half. Both have seen quite a bit of use.
I originally purchased the Viper PST 4-16 FFP (MOA) model on pre-order. I used it for a few weeks and ultimately decided that the optics were pretty fuzzy and called up Vortex. They offered to trade up for $100 bucks to the 6-24 because, their words, the 6-24x was clearer than the 4-16x. They were absolutely right.
Body
Both scopes weigh within a few ounces of each other, but the Vortex looks heavier and definitely feels bulkier. The eyepiece of the Vortex just seems huge compared to the Leupold especially with the illumination knob sticking out the side. Thunking my finger onto the metal gives the sound impression that the Vortex is sporting thicker metal walls. Win: Tie.
Turrets
The turrets on the Vortex are very nice and crisp - the Leupolds should take a lesson in knob crispness from Vortex. With very thick gloves, you have to watch the Leupold when you make adjustments. The Vortex knobs are SUPER high traction and it makes gloved work pretty easy, but the drawback is that the knobs have been accidentally moved many many times by putting into a padded case or sliding in/out of my Eberlestock lodrag bag. The indicator numbers and lines are bolder on the Vortex. On the Vortex, you are supposed to be able to see the magnification you are currently using just by looking at the indicator on the eyepiece without having to pull your face off the optic, but honestly, you can’t focus on it without pulling your face back far enough that you would be able to see it on the Leupold. When I bought the Vortex, I thought the Zero Stop would be awesome. I've never set it up because with all my other scopes, I got used to either just remembering where I was from zero or resetting after use. Even without it, you can see on the rotation lines where you are at in your adjustment range. Win: Tie. I really want to give the nod to the Vortex, but I HATE that they have rotated on their own.
Parallax Adjustment knob
I put this in its own place because I the parallax adjustment on my Vortex is very stiff. Stiff enough that I have griped about it a few times. I don’t have very tough skin on my hands and you can feel it on your finger and thumb after adjusting it a lot after a whole day shooting. Win: Leupold.
Clarity
Flat out, the Leupold is sharper than the Vortex. Its very easy to see that on a target 600 yards away, I can more clearly make out the lines with the Leupold. At 300 yards or so, I have a much easier time making out 22 caliber bullet holes with the Leupold. Somewhere between 900 and 1200 yards it becomes about a wash, because the Leupold is a tad sharper, but the Vortex has a bit more magnification. The Vortex definitely has a bit more chromatic aberration than the Leupold and is easy to see on a bright white target on a bright day, or with heavily backlit objects like right now where the house on the top of the hill is against a lit up cloud at sunset. The Vortex does maintain image clarity a bit better near the outer edges of the picture. I would say it’s a tie for low light work between these two scopes and both are pretty color neutral. Win: Leupold
Reticle
I hands down prefer the TMR to the EBR reticle. The EBR is marked in 2MOA and 5MOA and that’s it. The TMR has quite a few smaller subtensions that help with ranging. Another thing I like about the Leupold over the Vortex is that the reticle is open in the center. My biggest gripe about the Vortex reticle is the thickness of the reticle. The Vortex reticle is still thicker at 12X than the Leupolds is at 20X. The Vortex may be a bit easier to use at low powers than the Leupold, but I generally shoot on higher power in highpower matches.
Once thing that I think is worth mentioning that really bugs me about the Vortex is that when you adjust the magnification ring, you can see the reticle rotate ever….so…..slightly. You really have to be looking very closely at it near the edge of visible picture to see it. That just bugs me. How much does it skew? Not sure, it’s really hard to say, but I prefer my reticle to not skew for precision work. The 4-16 I had did this, and this one does as well. Win: Leupold
Illumination
Due to the Leupold reticle being so thin, I would recommend you get the illuminated model for hunting on lower powers. I never hunt or shoot at dusk/dawn so this is not an issue for me, but if I did, I could see it maybe being an issue. Since I never use it, I can't even comment on it on my Vortex. EDIT: I forgot to mention that right now the battery is dead on my Vortex. Its been that way because the knob somehow got turned to on at some point in the last two years and left on - further proof the knobs do grab on things.
Warranty
You can’t beat the Vortex warranty. Honestly, I’m not quite sure what the warranty is on my Leupold scope, but I didn’t buy it planning to use the warranty. So far, I haven’t had any issues with either so no dealings with warranty issues - I hope that it will stay that way for a long time. Win: Vortex
Conclusion
As far as value goes, the Vortex is a clear winner. I bought my Vortex and Leupolds in this order: Vortex, Leupold (6-20 ER/T), Leupold (8.5-25 ER/T). After I got my first Leupold, I couldn’t believe how well the Vortex held up for almost half the price. My second purchase would have been a Vortex had it not been mainly my issues with the reticle.
If you’re right on the edge price wise, go with the Vortex, you’ll be happy and use the money to spend time behind the gun. I went Leupold for my third scope after thoroughly evaluating both brands because it was in my budget and had an edge on a few things I cared about. But I'm not a fanboy of either - I'm in the market again and I'll probably be looking at another Vortex product (Razor HD) as well as everyone else's offerings.
I hope that helps you out, let me know if you have any other questions.