I would ask a different set of questions:
(1) if your mage had 15 spell picks for wiz-school spells L1-L6, which would you pick? (or 12 picks up to L5 or 9 up to L4 or...)
(2) what spellbooks can you find early enough that you don't have to 'pick' the spells during levelups?
(3) what spells do you need in order to get yourself up to Mage level 14 (that allows you 15 spell picks)?
The answer to these questions depends [bold]a great deal [/bold]on
- playstyle,
- party composition, current party level, and
- your own comfort level with combat
For example, if you have an Alchemist and a Mage, the mage doesn't need to get Noxious Fumes for a long time. If you have only the mage, it's an aweseome spell (both disabling and direct damaging).
It's a good idea to have 3 spell picks available when you reach L18 (for pure casters) so that you can grab all three L7 spells at once. At other level ups, the
right choice isn't so clear.