My party is just about complete. I've decided not to ship off for Spellhold right away, and to finish a few more quests than I usually do before leaving.

I'm going to have Nalia in my party from the start, so she'll be the highest level Mage in my party.

My backup mage comes down to two choices: Imoen or Jan.

Please note that I consider myself covered in the Thieving department with either one of these characters. All I'm concerned with is locks and traps.

Poll Information
This poll contains 1 question(s). 20 user(s) have voted.
You may not view the results of this poll without voting.

Poll Results: Help me choose a backup mage: (20 votes.)

Help me choose a backup mage: (Choose 1)
* Imoen - 60% (12)
* Jan Jansen - 40% (8)

For this matter I would suggest imoen. From what your saying i have reasont to beleive Nalia is your ONLY mage right now. Which means Jan wont do in my opinion. Jan simply isnt powerful enough for me. He lacks the stats to be a good mage. Hes a decent enough theif, but imoen does the job fine. Honestly I much prefer imoen to jan as she as a much better mage, and can handle the theif position. But if you do every quest in amn before you go to spell hold, take Jan, because all the experience will be extremely useful.

I voted to Imoen too. Once I finished the game with Jansen too, was not bad, especially the traps.
But Imoen is much more useful, especially in your case.

Strife: I haven't actually started this game yet. It's just geears turning in my head right now.

I've decided to go for a game, using no cheats at all, actually concentrating on doing the Jaheira romance correctly, and to it's completion.

I'm going to take Jan at the beginning of the game for the thieving (he's better than Yoshimo IMO), and then switch to Imoen when I leave for Spellhold.

Since there isn't much going on in the game between leaving for Spellhold, and actually getting Imoen, what I'll do is add Yoshimo to the party, dropping Jan (I'm an XP munchkin!), so that I'll be able to add Imoen when the time comes.

Imoen should only be a few levels (at most) behind Nalia, and having two Mages who progress through levels quickly is not a bad thing!

Since Imoen covers your thieving needs, she is the one to take since she's a better mage then Jan. Even if Jan has more experience, he needs alot more to be able to hold up to Imoen since she advances twice as fast in mage levels. The only reason you could ever prefer Jan is if you want to lay traps or pick pockets or you like his character more.

Well, obviously if you're just considering the mage aspect, Imoen is the way to go. She might be a little bit behind Jan when you first get her, but because Jan splits his XP's between thief and mage classes, Imoen will overtake him very quickly. That having been said, I usually only play the game with one full mage (either Nalia, Imoen or Edwin), and I usually find it good enough to have someone who is a multi-class (like Jan or Aerie) as a backup mage. I think the strongest combination (selecting two) is Imoen and Jan, but certainly can't fault you for taking both Imoen and Nalia if you aren't concerned with missing out on thief HLA's.