Am I more easily detected while sneaking with a spell equipped?

Spells only create sound when they're actually cast (unless you have the illusion perk which negates the sound). Spells create light whenever they're selected, and even more when they're actually cast. (This is very similar to the self-illumination via pipboy in Fallout 3).

Given that you can't score sneak attacks with spells in the first place, keep your mage hands in your thief pockets when going in for a sneak stab.