Without speaking for Trojan War (which I expect to be a great book and I'm really thrilled it's coming out), I can say that Testament uses the Vancian magic system and most of the tropes of the d20 system. The only significant mechanical variation is the use of Piety instead of alignment, but even that's optional, and the special situations in the book that address Piety (eg. spells that do damage depending on Piety level) usually have notes to suggest how they'd be handled with conventional alignment.
So if d20 compatibility was the tipping point on whether or not you bought Testament, please try to give it another look.
Scott Bennie (author of Testament)
