In my way of thinking there are three flavors of thermals:
01 - Dedicated Handheld spotters
02 - Dedicated rifle scopes
03 - Combo units
Items like the HD19A and EOTECH x320 would be examples of 01 ... they are only useful as handheld spotters. That's not to say handheld spotters are not useful. When stationary with a lanyard, you can quickly scan about and see critters and then release the spotter and shoulder the rifle and engage. And that might even be faster than flipping up a helmet mounted thermal.
Items like Zeus 75mm or Zeus Pro 50mm or Pulsar Apex xd50a or ir-hunter mk2 would be examples of 02 ... designed to be mounted on the rifle. The actual "scopes" often have optical magnification which is useful for greater precision.
I'll divide the "combo" units into two subgroups ... the helmet mountables and the clipons.
The problem with the clipons is they are pretty much 1x optical ... and when you use them in front of a day scope, if you use day scope magnification, you are zooming in on the pixels and quickly giving up screen size in order to see less pixels more closely. If having a rifle that is primarily a day rifle, but can be used in a pinch as a night rifle is a priority, then clipons make sense (to me). But if you want to optimize night shooting, then clipons don't seen to be the best solution (to me). I have used the apollo has a handheld spotter with a lanyard and then by using Larue rail clips to mark the spot, been able to put the apollo quickly on the rifle in the same spot. But there will be variation/repeatability issues with that approach though for hogs out to 150yds or there abouts that can work.
The final group I'll mention are the helmet mountables. These are the "do everything" bunch.
PAS-29/COTI
IR-Patrol
Armasight A-14
ATN Odin
You can mount them on your helmet while moving ... detach them from the helmet and use as handheld scanner while stationary ... and slap them on your rifle for a short range shot. But they are generally more expensive than the more dedicated solutions and not as good as weapon sights for various reasons. To me, the primary advantage of the "helmet mountables" is to be able to spot while moving.
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One of the best bang for the buck ways I know of to get well rounded capability is to get both the HD19A and the XD38A.
HD19A, 19mm, 1x, 384(50) street price around $1,900
XD38A, 38mm, 1.5x, 384(50) street price around $2,900
That gets you both a dedicated spotter and a dedicated rifle scope for $4,800
For a bit more you can substitute the XD50A
XD50A, 50mm, 2.0x, 384(50) street price about $3,400
I have the XD50A and it is the second best thermal image I've ever seen, right after the IR-Hunter Mk2 ... and I hardly ever nuc the xd50a, it nucs itself. It is 1.5 pounds ...