Wizard Nuke Question

I'm currently rocking PAL/WIZ/SHM at L24 and having an absolute blast (pun intended). I seem to get a new slightly better wizard DD spell every 4-5 levels, but the actual mana to damage ratio doesn't seem to change change dramatically.

This got me wondering if it would make sense to pick a spell I've got now (maybe Garrison's?) and drop a few motes into it rather than just constantly replace with the slightly upgraded nuke every few levels.

Has anyone done this and has it been worth it? If so, which spell did you choose to focus on?