Comparing Savant to Smarty is difficult. Yes they are both template engines, but Savant does not compile its templates. Nor does it have its own built in scripting syntax. It uses PHP.
Savant is a lot like using PHP amidst HTML, but it concentrates everything in an object. Of course, this doesn’t stop anyone from going “outside” the object and just writing PHP like crazy in their template, but it’s not designed to be sandboxed.
A lot of template engines are meant for non-programmers to write code, and I gather Savant isn’t like that — it’s just to let the programmer keep his presentation separate from his code. I’ll probably stick with Smarty, but Savant is nicely done.