I have the new SpectraPure UPLC-II and I love it; no float switch or optical sensor, just a pressure switch (and a backup pressure switch in case the main switch fails). Plus it's pretty silent (return pump is louder) and a 5-year warranty gives me a piece of mind in case it craps out by year 3+.
I had the Tunze 3155 but it's kinda finky; the switches/sensors get salt creep and causes it to overfill (but that's what monthly maintenance is for) and it's pretty loud (I thought it was okay but GF didn't like it, so I ended up with the UPLC-II. It works pretty well when I'm on top of maintenance (or not on a trip).