A flat stab IS a symetrical stab.
The only thing gained is lighter tail section, and therefore more authority because of overall reduced total weight, lower moment arm...
OK, there are other gains, such as structural integrity...

Stan Douglass hit it on the head.
Less drag on larger models.
A 60" span is a considerable sized plane. Flat stab would work just fine. Built-up stab, (symetrical), would reduce overall weight, but if you need to add weight in the tail to counteract it, then the advantage would be lost.
Reality, I think it won't matter enough to get your shorts wrung out on this.
Just remember that aerodynamics is NOT proprtionate to scale. Smaller planes can get away with so much more deviation than larger planes, as Mr. Hanson has said in so many other posts.
60" is near the limit though where I would start to consider built-up stabs.
(My personal opinion)

Bob