When to dual class a Mage to Cleric, or a Cleric to Mage, is based on which of the classes one started out as. The second class is the one that will get access to the higher level spells, more spells per day, and the High Level Abilities.

So it boils down to: Do you want to be a Mage with some Clerical abilities, or do you want to be a Cleric with some Mage abilities? This is especially important if you play with the level cap in place, of course, since you can only amass 8 million xp through to the end of ToB.

One has to look at the spell charts for known spells, and number of spells one can cast per day, to make any sort of decision on this. For example, if one wants to be a Cl>MU that has access to 7th lvl Cl spells [at least one per day] then you have to get to Cl 14th lvl before dual classing. If you want to be an MU>Cl that has access to 9th lvl MU spells, then you would have to advance to level 18 as an MU before dual classing.

Whether or not someone is willing to gain a million or more xp as their first class, before dual classing is of course up to them. A Cl to MU dual class could do it, i.e. get access to the highest level Cl spells, one once per day anyway, after "only" 1,350,000 xp. Whether or not becoming a 1st level MU in, say, the Underdark, is something someone wants to do is also their decision, of course.

Personally, I'd just play a Cl/MU multi-class, if I wanted access to both groups of spells and abilities. Actually, I'd just take Aerie instead, but people play the game differently, so whatever suits the individual is the "right" answer for them. :)

Half Elf is the one out of two available choices, for multiclass Cleric/Mage. This combo is best when multi. Because this way, in ToB, hitting max level, means you are able to pick all of the HLAs, both Mage's and Cleric's. Also, this way, you can cast both 9th level wizard spells and level 7 divine ones, without sacrificing anything else, besides some slots in book/scroll (less high level spells of both classes). If Aerie had a bit higher wisdom, though, she would be better overall. Elves have higher racial resistances than Half-breeds. A cleric mage is best at neutral alignment, because this way, he gets all priest spells, and gets to choose between good/evil deva/planetar.

Aerie has 16 wisdom, and has good alignment, which prevents her from learning all priest spells. Also, she has 16 intelligence, which means a bit less spell slots for mage spells, and more frequent failure at spell scribing (if you play in AD&D by the rules difficulty setting). The only thing Aerie is better at as a cleric mage, is her being an "elf", because racials are stronger than the half-elf, the only allowable race for this combo for player character.

I mostly want to be a cleric for RP reasons, but I started as a mage and don't want to mainly be a cleric. Though admtitedly that call down lightening spell is very sexy.

Also I'd take Aerie but uh, she doesn't like Jaheira too much.

Aerie is very sensitive about being bossed around, and Jaheira is very bossy. Don't worry though, they get along just fine unless you are on a double romance track, and even then nothing will come of the bickering, party-wise. Aerie should never be underestimated if she fits in with your party dynamic.

Yeah I got both Aerie and Jaheira in my party. They bickered until Charname picked one of them (Aerie in my case). No more conflicts afterwards.

And Aerie is probably my 3rd most powerful character and useful in almost every situation.

Maybe that was my problem. I guess I was being a little to uh... kind to Aerie.

And oh man am I a nub. I just realized you can dual class from the get-go and have both classes level up at the same time. Is that better than dual classing later?

Yes. Multiclassed characters and Dualclassed characters have both their own prons and cons. Generally, though, BG2 favors dualclassed characters, and Icewind Dale 1 multiclassed (because in ID they can reach max in both classes when being multi).