The 190s actually take IDE hard drives. The only display options on the 190 are passive matrix (grey or colour), if that's important. The 500-series is available with all types: active/passive, b&w/color. The 500 series can also take PCMCIA cards, if you manage to locate the relatively rare expansion card cage dealie for them.
5300s are pretty nice machines, after the power ports are fixed and the flaming Sony LiIon battery fiasco is ignored (they've used non-fire prone NiMH batteries since the almost immediate recall in '95). I like my 5300s. OS 8.6 on a 5300 is pretty nice: responsive, relatively modern (you can run OS 9 if you REALLY want to), and excellent for use in harsh lighting conditions (with that b&w screen). The 5300ce is pretty nice for its higher resolution screen and marginally faster processor.
The only Duo you're likely to find working properly when you get it is a 2300c. This is because the version F keyboards on those things are remarkably improved over the models used in previous Duos; they're still rather weak in comparison with other keyboards, though.
The 2400 is nice when you find 'em. They're kinda finicky, though, and they're less than easy to get apart. Failure of the power board is not uncommon, and results in a permanent Green Light of Death and/or Click of Death (from the speaker, not the hard drive). Replacing the hard drives in those things requires disassembling the ENTIRE computer. They're blazingly fast compared to their predecessors, though. When you find a good one, they're more than worth it.
As for the PB150, I don't much care for them in average use. However, they're a little more rugged than other models of the same vintage (that's one bonus of being so stripped down), and finding replacement batteries (or re-celling your own) isn't all that difficult. As others have noted, finding RAM, particularly the 90˚ angle adapter required, is often a fruitless endeavor. It's best to buy those things with the RAM already upgraded beyond the 4MB base.