There are pros and cons to each.
The sight picture of the IR Defense (IRD) products is the best IMO. It just has a very sharp, detailed image. On the flip side, it has two things that I wish they would change. I wish it had auto-NUCing and that the video out was always on. You get used to the operation and learn to efficiently work around it, but still...it just shouldn't be that way. If it had those two items, no one would be anywhere close IMO.
If you decide to go with a 2.5X instead of the 4.5X, I would consider the REAP IR. Same software, but a smaller and lighter package. The MKIII has turret controls, while the REAP IR uses a joystick. I haven't fully settled on my preference, but at this point I think I prefer the turret.
I also have the IR Patrol which is much like the REAP IR, except it is 1X mag. I wouldn't use it as a scope, but have used it as one of four thermal monocs.
I have a Pulsar 50XD. It has some nice features for an inexpensive scope and is backed by Sellmark which has the best customer service in the business. It offers the choice of manual, semi-auto and auto NUCing and the video out is always live. The PiP option is nice too. There are two deal killers for me with the scope; it doesn't come in 640 and it doesn't have enough mag to be used at reasonable hunting distances.
I have an ATN 640, 5X. For whatever reason, I have just never fallen in love with this scope....and it is the most expensive original retail price thermal product I own. It weighs 3.1 lbs. for starters. But the sight picture has never grabbed me like the IRD. That is saying something because I was new to thermal when I first used the ATN. On the other hand, I had quite a bit of experience when I first looked into an IRD. The ATN also requires you to turn the video out on each time you power up and has manual NUCing. Sadly, it just hasn't done it for me.
I just acquired my first Armasight so the jury is still out on it. It is a Zeus 3X - 24X, 640, 60 Hz, 75mm. The video out is always on and it has auto NUCing. At this point, I like the IRD sight picture better....but it is close enough. I sighted it in today and the process was simple. I haven't found anything not to like so far, well maybe buttons you use for operation. I like the turrets and joystick on the IRD products better.
So, in the end, the issues I have with the IRD are manageable and the sight picture is still my favorite. If I were going to consider something else, I would suggest you take a look at the Armasight before you put down the big bucks. You may not choose it, but it gives you something to compare to.
Back to the mag thing for just a sec. Everyone likes something different. I think sometimes it has to do with what we get used to, adjust to. Regardless, if you are buying thermal you are planning to hunt at night...which prolly means hunting hogs. Your environment makes a huge difference in scope choice. Short range (50-75 yards) and wooded, you can get away with a 3XX core and lower mag. 100 yards and more, open areas, a 640 core and more mag helps....it helps a lot the further out you go. Sometimes we think we will always hunt one way, then things change. I was hunting shorter ranges, closed in and now I hunt longer ranges, more open. Regardless of anything, I have a hard time with that ATN 5X staying on a running sounder sometimes. 2.5X - 3X works much better for me when they get their go on...
These are my experiences. Others will have their own input...and I appreciate them all. Never stop learning.
I hope this helps.