I use both, and will try to give and apples to apples comparison of the two (ETA: meaning comparing two with more similar power ranges than what you have listed - a 10x top end and a 14 power - Leupold has a 4.5-14x40 with the older style turrets that is in the $700+ dollar range).
Leupold Mk 4 3.5-10x40 mil dot M1 illuminated on Rem 700P .308
Nikon Tactical 2.5-10x44 mil dot non-illuminated on Armalite AR
Now both of these scopes were bought new maybe 3-4 years ago, and since then, things might have changed. But both were bought pretty close to the same time, and have been used in similar training and conditions.
Both have seen quite a bit of banging around in their respective soft drag bags - neither has been kept in a hard case. They have been thrown around in the arms room, and tossed into vehicles. Both have been shot a bunch - the Nikon more just because it's easier to load and shoot the AR - but it has less recoil. Neither has lost zero ever (and in fairness, we have 5 or 6 leupolds at our PD, and I only have the one Nikon, so overall I'd say the Leupolds have shown themselves to be more durable just because the number we have seen take similar abuse). The Nikon seems every bit as sturdy as the Leupold just based on handling and shooting. Both have been shot in absolutely terrible weather - freezing cold to 100+ temps to pouring rain. No problems ever.
They are about the same size. The Luppy is 0.4" shorter and 4 oz lighter. The Luppy has a 4mm smaller objective. The difference between the two is almost unnoticable.
Both have similar baseball-type 2/10ths mil dots. The reticles are very similar. Both are second focal plane, and you mil with both on 10 power (which I'm fine with). Frankly, since most of my work is done at such close range, I like having a big reticle when dialed down and up close. Easy to see for me, and I frankly have never milled anything in real life - only in schools. Milling is pretty close to useless from across the street - but that's another story.
Both have 30 mm tubes and have parallax adjustments on the left side. Both operate pretty much the same. The windage and elevation turrets are similar 1/4 moa finger adjustable without covers, but the Nikon had a few differences. The leupold M1 turret is taller - or the Nikon is shorter - depending if your glass is half full or half empty. The Leupold is 15 minutes per revolution, the Nikon 10. I was used to the Leupold, but later found I kinda liked the 10 MOA per rev. It is easier for me to use. It's just easier for me sitting behind the rifle to add 10 to what is showing than 15. It's just easier and faster for me to do the math. It never goes over 9, so you don't find yourself adding 12 to 15. To me, simple is good. I admit my shortcomings, and think under stress my math skills might be pretty bad.
I also like the 2.5 power low end compared to the 3.5 power - just a bit wider FOV up close (42'@100 yds compared to 29.5'@100). Both are comparable in their magnification adjustments - neither strikes me as harder or easier to use. Neither turns the eyepiece when you adjust the magnification, so your scope cap doesn't have to be removed to keep it from binding the bolt (like the Burris and I think the nightforce - but I don't have the latter and have just used them a few times).
Optics-wise, the glass on the Nikon is a bit better to my eye. The image is brighter and it is a tiny bit easier to see details. At night, it is a bit easier to see things with the Nikon for me at all magnifications. I think half of our leupolds have illuminated reticles, the others don't. The one I used most had it, but I found it of little use, frankly. I found that if it was so dark I couldn't see the crosshairs, I had no chance of being able to ID a target. Plus, I found in really dark conditions, the illuminated crosshairs showed up on the bad guy end a little too well. Added to the fact that since I work in a city with lots of ambient light at night (unless there is a city-wide blackout), there is just little use FOR ME for the illuminated reticle.
Of course, to me the price of the Leupold was much better. They were free along with the rifle it was mounted on and all the ammo I needed. But for my personal rifles I wanted to experiment a bit with other similar brands - and the Nikon has impressed me most of the ones I've tried (Nikon, IOR, and Burris XTR).
Were I to do it again, I would probably go with the Nikon again - even if someone else was putting up the money. The slight differences between the two - wider magnification range, slightly better glass, and 10 moa/rev turrets - make the difference.
I have not played with the Nikons since they changed the name from the Tactical to the Monarch X 30mm. I have heard that they made some changes - but I have nothing to substantiate that. I also have heard that Leupold has made some changes to their Mk 4 line - but I also don't know what they are.
But I'd say your pretty safe with either. Both have good warranties (lifetime), and both are well established companies.