Since there's an option to switch Xan into a Sorcerer, I'm wondering which NPC mod character makes a better Sorcerer in BGII.

TIA

Tashia:)

I've played neither but I believe Kelsey is meant to have more character interaction, but poorer early level spell selections (1 or 2 odd spells in order to be consistent with the idea that he had little knowledge of magic upon realizing he was a sorcerer).

Tashia was also pretty good, if not a tad overpowered (she had a figurine that could summon a cat which was probably pretty-well balanced were it not for the time-stop resistance)

Tashia.

Does Tashia have good spell picks? How is the romance?

It has been too long since I palyed her to answer about her spells.

I will say it was one of the most fun romances I have played.

Spell picks were pretty good, however they were a bit different to what I was used to when I played it, so I changed the around a bit in SK (this isn't to say the picks were bad, I was just relatively unexperienced at the time).
The romance was top-notch but I didn't really like her portrait (ended up changing it to something a bit sexier).

I think the newer version has some improved portraits on offer. Quite liked the romance but got hit by a nasty bug in it which stopped it dead. Haven't checked recently whether it has been fixed.