Turck is only marginally faster than the Zend optimizer (the full version, not the free one that comes with PHP nowadays), and it isn't reccommended to have both installed (though you can)
Turck actually stat's the source every time, to see if it changed. I haven't had any issues with it using old versions, though I used to with mod_perl (found the way to change it). You can even tell it not to cache specific extentions, too.
I think that layered will take pretty much any payment, and they have
a pretty sweet blowout going on now.
Whatever you pick, goodluck and godspeed
