WoW newbie looking for advice

[Bob the Beholder]

Ugh, it's tough to decide what benefits me more, +31 more spell damage (which will make it +51 total by level 35) or +28 Spirit and +23 Intellect. The downtime with the spell gear is already noticeable, and will be even more so by next level when I get the aforementioned +51 spell damage and even weaker stats, as I exchange +stats gear for +spell damage grear.

It seems that I kill somewhat faster with the additional +spell damage (+20 is there regardless of anything else), but the downtime is kinda getting annoying. Guess I've been spoiled by Spirit Tap.:)
Probably couldn't play a Mage after this ...

Think about it like this--the more spell damage you have, the more mana efficient your spells are. The increased spell damage does not increase the mana cost, so the more you've got, the more your mana is worth. You'll have less mana than somebody rolling in full int gear but you'll be doing more with it, and the increased damage will kill mobs faster, requiring fewer casts, saving you mana.

... it just sorta takes a bit more than +20 to make a big difference. Hit up the AH for two or three more pieces if you can, and you'll be blowing stuff to hell. In the 42-49 level area, there's a tailored shadow set you can use, called Shadoweave.