I've sprayed a bunch of bows with Fullerplast, then I tried Thunderbird. I won't use Fullerplast again unless they quit making Thunderbird. It's more durable and I like the look a little better.
They're about the same as far as mixing, clean up, spraying, etc. Both are sandable as their own fillers. Both harden easily over oily woods. Fullerplast dries faster.