What's better thing to do? to pay bodhi 15000 gp or Gaelan (end of chapter 2)? Does the story depends on this action?

Well, you asked for it, so here are some spoilers:

1. You will pay 15000gp either way

2. If you work for Gaelen, then you will have to fight Bodhi and her vamps. If you work for Bodhi, then you will have to fight Gaelen and the Shadow Thieves. So, you have to ask if you fancy your chances against the vampires or thieves. You will get about the same amt of EXP either way. If you have an item that grants Neg Plane Protection (like the upgraded Mace of Disruption), then the vamps might not be so difficult. One other point, is that if you do side with Bodhi, you will wind up with some better items off the thieves' carcasses.

3. Either way, you will get to Imoen.

4. Either way, you will eventually fight Irenicus, and the later chapters will be the same.

5. Your alignment doesn't really affect your choice-even if you are 'Good', you can side with Bodhi; or if you are 'Evil', you can side with Gaelen.

I prefer to go with the Shadow thieves, you do tasks either way, but

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When Bodhi turns out to be Irenicus' sister, and hwen you have to finally hunt her down in her lair, if you worked for the shadow thieves, you can ask Aran Linvail for help and you get a couple of assassins to go along with you.

Also if you sie with Bodhi you'll lose the Blackmarket Thief and the Merchant at Mae'Var's leaving the hard-to-find Roger the Fence as only
merchant accepting stolen goods.
With a high level cleric the vamps are no problem. Just turn undead and wohoo!!