I use the heated larger unit. It's not very old but it's already paid for itself with the amount of stuff I end up cleaning. I would not be able to get by with the smaller unit, but this one is ok.
I use the small Pyrex rectangular dish things that cost $4.95 at the local Wally World to store my cleaning fluids, they fit perfectly in the larger model. Read up on how to do this, you can damage your unit or have poor cleaning if you do it wrong.
I don't leave the main water in the unit long, I don't think the SS basin is really the highest quality metal and one spot developed a slight brown marking that has me worried about rust if I always leave water in it. But, because I keep my cleaning liquids in the separate Pyrex containers, it's no big deal to dump the plain water which only has a little detergent to break up water tension.
I like the heat, but if you forget the unit is on, the heater seems to keep running after the cycle kicks off and makes odd noises as it heats, so you need to remember to go turn the heat off after the ultrasonic cycle is done.
With the coupon, it's a no brainer for me, but of course it's not expected to last as long as the higher quality units that also cost several times more.
I used it with some cleaning fluids for firearms that seemed to melt some of the plastic on the lid, just as an FYI. That is an issue that the all metal units don't necessarily have.