A3- visible green and IR laser plus IR illuminator. Visible and IR laser are "slaved" together so you can go out and zero the visible during the day and the IR laser will be on also. Biggest complaint I hear is people saying they have no range to shoot at during night time. So for some this is the best option.
I2- various options there- red or green visible + IR laser. Not "slaved" together so your doing two zero jobs, no illuminator. I have had an old skewl red visible/IR version of this on my main training rifle for 6-7 years now. That rifle sees easily 10-15K rounds a year. Other than losing the caps for the laser output, never had a problem with the unit or zero.
There is an I2 variant Steiner number 9007 that has an IR laser and an IR illuminator. This is a good middle of the road price range option for those that don't want/need a visible laser. I have one of these on another training rifle that sees less use, maybe just a few thousand rounds a year on that rifle. I like the IR laser dot within the IR illuminator feature- illuminate and check an area at distance while being ready to shoot as well.
A4 is supposed to be coming out in late March, it's a big honking deal and pretty damn pricey. Honestly I'd go with an A3 and if you felt the need for a white light, add a light on yourself and put the cash difference towards a training class or practice ammo.
Most basic is the OTAL-C which is just an IR laser, no other bells and whistles. This one takes AA battery versus CR123 if that matters to you also. Very simple, only thing I've seen new people mess up is leaving the caps on, other than that it's really hard to do something wrong with it. Something to consider also is that if your outfitting a rifle for someone who potentially doesn't train as much or isn't going to probably be as familiar with their weapon/equipment- the OTAL-C is just IR, very simple to use and a lot easier to avoid any ND of say a visible laser stream like an inexperienced user could easily do with an I2. Something to consider.