I've used the Amsoil ATF in my car and it smoothened out my shifts. However, there is yet another price increase from Amsoil due to crude oil prices, this takes place July 1, 2006. If cost isn't an issue, I believe it's about $9 retail ($8.65) and about $7 with preferred customer ($6.60)for now. Only time I've seen Mobil 1 was at Auto Zone and it was about the same price as Amsoil's ATF at the preferred customer price.
There are good on both sides, if price isn't a major issue, I would go for Amsoil. Have not heard any bad about the Mobil 1 ATF though. Since you're already using a synthetic ATF, you may not notice too much of a difference with the Amsoil.
How often do you plan to drain/flush the system?