I've owned both! They both sound great, but have different voices for sure.
The YYZ doesn't have a true clean channel, it has a processed clean that has a significant roll off on the high end, and the dirty channel is supposed to cover the upper register of your tone completely. It also has a bit of a baked in tone to the high end that is always there, you either like it or you don't.
The Sheehan Ultimate has a 100% clean channel, and then a separate drive channel you blend in. The YYZ has EQ, the Sheehan has the loop feature and a good usable compressor built in.
overall, the Sheehan pedal worked better for me, and that's what I still have. I use it as a super subtle drive with the drive knob at a near minimum level, and the drive channel only loud enough to change the character of the midrange on the clean channel ever so slightly. It's my favorite dirt sound since I first jumped into the whole pedal black hole all those years ago, right along with the Lomenzo Hyperdrive which is nearly the same circuit but has a much clunkier form factor. I digress, but that's my opinion on both boxes. Feel free to ask any questions if you want more detail