Is there ever any situation where Fewer Gylphs is prefferable to More Glyphs?
Monster classes, maybe. Even then the attack bonus boost is pretty big for vampires and gorgons so I can't imagine using it for them.
you'll never convert all your glyphs
Well, there are warmongers, and many classes start with an extra glyph so they can convert all except one for the same effect.
so I'm willing to bet you get more CP from More Glyphs anyway
If you convert 3/4 glyphs on fewer glyphs then you get 450 CP.
If you convert 4/5 glyphs on normal glyph then you get 400 CP.
If you convert 5/6 glyphs on more glyphs then you get 400 CP.
For T3 classes:
If you convert 4/5 glyphs on fewer glyphs then you get 600 CP.
If you convert 5/6 glyphs on normal glyph then you get 500 CP.
If you convert 6/7 glyphs on more glyphs then you get 480 CP.
So fewer glyphs is still ahead overall, but not by much. There's very
little benefit to be had here if you don't convert absolutely everything, and even then it's not enough to merit using this instead of the booster preps, and reducing the chances of finding useful glyphs just isn't worth this tradeoff.
More glyphs is definitely going to need a buff.
Possibly wizards because you can find all the glyphs before you start converting? You'd need to be somewhere where there's a lot of junk like spoons to make it work even then...
In an average scenario, wizards will curiously get more
CP from more glyphs, since each one contributes +10 when converting an item. Since a typical dungeon will see you convert fewer than 8 things they will probably never become fully depleted until the end-game. For front-loaded conversion races like elves, humans, or orcs that's a non-issue.