Not a huge difference either way. Both offer sterile transit - no need to clear customs in Canada. No need to re-clear security to go US>INTL. Both have US pre-clearance, so no need to go through customs when you get back to PHL as you'll do it during your layover.

The biggest difference is YYZ is a bigger hub with more options if something goes wrong and you misconnect. But that's of marginal value.

YYZ was somewhat of a mess for international arrivals earlier in the pandemic, but seems much smoother lately (there's a thread on that topic on this forum).

I would choose primarily based on schedule and price and not worry much about which of the two airports.