After my great delight at trying tmsu, I was willing to try out tagsistant on Eric Davis‘s suggestion. And between the two of them, I think Eric is right: tmsu might be a better fit for me too.

I can see similarities between tmsu and tagsistant; both use directory trees to arrange tags, and to show combinations between them. tagsistant seems very intuitive when it comes to finding mixed tags, with a simple plus sign showing combinations, and so forth.
tagistant “tags” files by “copying” them into a tag folder, which means adding an entire subfolder is a lot easier than tmsu was. And you can tag a directory itself, without necessarily applying the tag to files inside it. I can see where that might be preferable, and the home page suggests that will keep the database slim.
tagsistant is also file-manager friendly, and I suppose the same thing could be said about tmsu. Once I had a few files tagged and cross-tagged, I could work my way through the directory tree with Midnight Commander, and see how files were arranged between tags.
Probably the best part of tagsistant was the setup. The home page shows you in simple steps how to create a tag system, tag individual files and work the query process. It’s a very comfortable introduction.
My complaints against tagsistant are also simple and fairly straightforward: Perhaps biggest, there’s no way I could see to query the tag system without working through the folder tree.
With tmsu I could just ask it outright what tags were applied to a file, but I don’t see an analogue for that in tagsistant. Please point it out to me, if I’m just being dense.
Second, I see no expedient way to apply multiple tags at a time. The copy-file-to-tag motif is an interesting approach, but it doesn’t lend itself to adding three or four tags at once. I prefer tmsu’s approach, of just listing tags in quick succession.
That might go back to my first, and this last point, just that the tree structure for tagsistant is a useful format, but can be very cumbersome without a file manager. Tab completion helps, but each query up or down the tree is going to require some backspacing, correction and possibly even retyping. My advice? Pick a quick file manager. 
(Of course, maybe if you could combine this with something like commacd, you might have a very powerful combination. … π )
tagsistant is by no means an unusable tool, and depending on your files and tags, you may prefer this over other options. Definitely look through the advanced documentation, because it will help you with a lot of tagsistant’s finer points.
As for myself, I’ll stick with tmsu, unless one of Eric’s other recommendations wins me over. … π