acdsee is great at viewing thumnails accross multiple arbitrary directories - it basically has checkboxes and lets you toggle on and off various directories to view simultaneously.
i don;t know if thumbsplus is so flexible, but you can easily say, show me every image in this folder AND ALL SUBFOLDERS.
one of the reasons i say its good at huge image collections is its ability to let you ask it to catalog/database thumbnails for huge directory trees on demand, which can be really useful when you have 10 thousand images and you want them scanned while you go have a meal. i couldnt find a way to do this in acdsee (though it may be there), which means there is an initial delay the first time you visit any folders. thumbsplus is designed to queue up such operations and show you nice progress, etc.
but there is only one real answer:
install trial versions of them and play with them and see
