The reason I went with the 125b is to somewhat satisfy my OCD. 99% of my pedals are 125b and I like it when they match. No other real reason to be honest.
It's about height for me. I also try to stick with 1590bb2 over 1590bb if I can. I use so many 125b in general that the few things I've put in 1590b seem harder to step on.
I'm standardising on 1590N1 myself. At H-120mm across the board you can use...
1590N1 / 125B (W-60mm)
1590BB BB2 BBS (portrait W-94mm)
1590Q (W-120mm) — 120²! — for those builds that were designed for BB-landscape
1590XX (W-145mm landscape)
1590DD (W-188mm landscape)
1550M (W-100mm)
1550E (W-171mm) ... a tad tall with a depth of 51mm, but some builds may need it.
Maybe:
1590B3 (116x77x37)
I think the 1590B3 could be snuck in on the 120-paradigm, if/when needed to squeeze width-wise on a board (otherwise I'd just go 1590BB2).
There just really isn't enough of a space-saving with 1590B for me to bother with.
I have an all 1590A board planned just for the fun of it, not for the practicality — for practicality, I'm considering an all 1590B board, ie taking all the 1590A PCBs and putting them in 1590Bs with top jacks and the board-size would remain the same yet I'd have room for mods and extra controls on the 1590B enclosures.
I was going try my hand at bending a few custom-enclosures, 120x66 (B-width/BB-portrait-height), but my friend gave his sheet-metal brake to his son so accessibility has been greatly reduced (he lives out of town, busy schedule etc).
Ideally, I'd have a 1590B Q-TUNE, too — maybe when funds permit.