Haven't decided yet
Tashia: She is funny, but her reaction before going to confront Irenicus at the Tree of Life was a bit disappointing.

In ToB, she has some funny dialogues with Imoen, and Immy seems to appreciate Tashia's efforts to cheer her up.
The main reason for wanting to keep Tashia is her spells: She's level 22 and has lots of Chain Contingencies, Death Spells, Dragon's Breaths, etc. There is no chance for any other mage in the party to get *her* stock of spells till the end. Of course, she is vulnerable to fighter's attacks but her usual Chain Contingency (Stoneskin, Spell Deflection, Mass Invisibility) seems to work quite well at the start of a battle, making things easier also for the rest of the party.
Imoen: Not only she is the only thief in the party, but, believe it or not, it's the first time I have taken her with me. So, I'm interested in her progress through the game and in her dialogues with Sarevok as well.
Keldorn: From a RPG point of view, he has been a true friend to me, and I'm really very fond of him --he's probably the only NPC of the game whom I deeply respect. His True Sight and Dispel Magic special abilities have saved our lives many times so far. I believe it's just *unfair* to kick him out; not after all these battles he has given for me and my party.
Perhaps Sarevok is far better in the battle field than Keldorn, but with Ascension installed, fighters are as much needed as mages. This means that if I replace Keldorn with Sarevok, there would be just two pure fighters in my party.
And so, we are back at the beginning. If I hadn't Ascension installed things would be easier, perhaps. Maybe I would have to say goodbye to Tashia, hoping that Imoen and Solaufein would be decent enough as mages? Honestly, I don't know.
Cheers
• "You cannot pass."..."I am a servant of the Secret Fire, Wielder of the Flame of Anor, You cannot pass. The dark fire will not avail you, Flame of Udun. Go Back to the Shadow! You cannot pass."
Gandalf the Grey