Thing is, Sif seems to give lots of semi-random books while Vehumet gives you only a couple of books that are hyper specialized in nuking shit and then after that pretty much leaves you on your own. I love Vehumet, he's my boy. But he's the God of Destructive Magic, and that's pretty much all that he does. He's really, really good at it and I'd never regret taking him as a conjurer for even a second. Sif Muna, however, gave me an absolutely stupid number of books for my generalist Wizard (Level 19+ Spellcasting can let you dabble in most low level spells pretty nicely) to play with, AND the ability to forget spells at will. Being able to memorize and forget a wide breadth of situational spells willy-nilly was pretty nice, particularly since it gave me the chance to try out just a ridiculous number of spells for the first time. I learned more about how various spells work from that single Deep Elf Wizard than I have with all my other casters combined. Plus, eventually Sif gave me some book that let me cast Alter Self, which I used to repeatedly mutate myself until I gained natural Teleport Control and some kind of natural Repulsion Field(!). Granted, I also ended up with a mildly deformed body (couldn't wear heavy armor) and mild reduced strength, but considering that I was a freaking Wizard, it didn't bother me much. It took a while, largely because I miscast it a lot at first, but it wasn't exactly hard to do either, particularly since Sif protects you from the negative effects of miscasting. Miscasting, btw, is bad, because it can eventually cause you to mutate, except when you do, it's almost always something bad; a fact I learned the hardway with a Vehumite Conjurer who really should have just stuck to throwing fireballs rather than toying with out-of-his-league Divinations.
Basically, what I'm saying here is, both gods are awesome. Vehumet melts faces, while Sif lets you melt your own face and then mold it like putty until you can spit venom and teleport at will.
Also, I pretty much exclusively play casters now since I suck something terrible with melees.