I own the tmb-100, and it's a fantastic bass. I took it on the road and it never failed me or made me feel that it was lacking.
I've played the 600 and it's also awesome.
The main differences are a gloss bound and blocked neck (but I believe the 300 has that as well. The 100 has a satin finish and dot inlays), Ash body (as opposed to mahogany on the earlier ones and now poplar), Nordstrand designed pickups (stock Ibanez on the 100 and 300), and an active/passive switch (100 is active only, not sure about the 300).
The neck dimensions, tuners, and bridge are the same.
I prefer the 100 strictly because it was really cheap (paid nothing after trading a few old pedals) and I modded the hell out of mine. Hipshot ultralite, hipshot kick-ass bridge, passive conversion with an EMG GZR pickup and taking a Dremel to the body for better upper fret access.
If I was already $600ish into a TMB600, the mods would've put it closer to $1000, and I would've been less inclined to experiment and cut it up.
Anyway, that's probably way more info than you're looking for, but I hope it helps. Here's a pic of my modded tmb100, not that you asked, but I love it.
I'd like to get the maple necked tmb100m sometime soon.
Here's a video of it in a live setting. Admittedly a frustrating video to watch, but maybe it'll help.