1600x1200 and 1920x1080 are *close* to the same number of pixels; playing with the calculator both would *technically* fit into 6MB of RAM at 24 bit color and 8MB at 32 bit (again, most video cards today use 32 bit words to store a 24 bit color framebuffers; the extra ram is sometimes wasted, sometimes used as an alpha channel/overlay, something. I don't know what the Mac drivers do; I *think* Mach 64/"Rage LT" supports a packed 24 bit framebuffer but there's a performance hit for using it while I don't think Radeon does at all. Rage 128, which the Pismo has, no idea.) So... what happens if you open your laptop and try to dual-head it, can you still use full color depth? Even with packed 24bit pixels there's not going to be enough RAM to do that with 1920x1080, at 1600x1200... you *just* barely should fit. That's one way to determine if it's packed or 32 bit.
In any case, if I *needed* a native OS 9 machine I'd still take the DVI Ti (if it wasn't a total basket case) over a Pismo or Lombard simply because MOAR is usually better. The CPU is faster, bus is faster, and the video card is *oodles* better, particularly if you have any desire at all to run OS X on it. The only "legacy" port the Lombard has that the Ti lacks is its oddball SCSI port so it's not really a much better "bridge" machine.
Any of them would probably end up anchored to a desk, however. You carry these old dogs around they're just not going to last.