Thanks to Kevin S, for this excellent rundown:
I have a Tecsun PL-450, and yes it is a bit large to be an Ultralight, but it is still rather small. Its features include two bandwidths on AM (same filters as the in the PL-600), but doesn't have the SSB that the PL-600 does. Doing a quick comparison with the much larger Sony 7600GR and the smaller Tecsun PL-310 just now: AM: sensitivity on the PL-450 and the 7600GR are essentially the same(!), and the selectivity choices on the PL-450 are better. The PL-310, ostensibly the most sensitive Ultralight, was jealous while listening to the other two pull in weak stations (i.e., it was noticeably less sensitive). However, with night-time conditions, and especially with a passive antenna such as the Terk Loop, the DSP bandwidths on the PL-310 would allow it to be the overall AM leader IMHO. FM: the PL-310 had a good laugh here, being easily more sensitive and selective than the other two. The PL-450 was doing OK; the 7600GR, with presumably very poor filtering and image/mixing product problems, couldn't even detect the weak signals that the PL-310 had a good hold of and the PL-450 was struggling with - quite dismal.
SW: using the whip antennae, the 7600GR held the edge over the PL-450 in sensitivity, with the PL-310 not that far behind. With so few SW stations around, the filtering advantage of the PL-310 (and to a lesser extent the PL-450) is probably not that important. So, as a travel portable, the PL-450 would be a good all-around choice, although without synch or SSB, there are some limitations depending on your desires.
A PL-600 is not that much more expensive and is likely more sensitive on AM (and has SSB), and the PL-310 costs about as much and offers a lot as well, so the niche that the PL-450 fills is perhaps a fairly narrow one. The PL-310 and PL-380 differ in that the 310 is more sensitive on both AM and FM, but I like the ergonomics and lack of "soft mute" (decrease in volume when tuning off-frequency) of the 380 better. So, both get a fair amount of use, and both are substantially better on AM, LW and SW than the PL-300WT (aka Grundig G8).
One more thing on the PL-310 and PL-380: they both have internal "birdies" (tones) on AM, which show up on various frequencies, especially on the high end of the band. With a passive loop helping out, the tones are much less pronounced, but they can be irritating to some. With a really big signal level, such as with local stations or with a high-power passive antenna such as I use, I don't hear the birdies at all. The PL-450 doesn't have such problems, so is to be commended for that.