I've tried them both along with a few different lemmy instances and found .today on lemmy is my best fit.
Since others have covered the technical angle, I'll say I prefer lemmy from a social perspective. My instance doesn't block other instances, banning users only for extreme cases. I prefer to choose my experience for myself without being ideologically guided by someone else's values.
For all the criticisms of the lemmy developers, their biases don't make it into the code. Instances are run according to the admins and user's wishes, which allows .today to operate as openly as possible. Piefed, on the other hand, suffers from the biases of its developer, who not only blocks users but also instances of admins who don't align with them. They have passed along their ban list to other admins as well, getting users banned for disagreeing with them. That's not the behavior of someone I want running my platform of choice.
Along with that. I, personally, don't like the idea of reputation, low score flags, karma or any system based on the votes of users, since they can be easily manipulated. It feels like Piefed is trying to implement a more intense karma system, which was one of my main problems with reddit. I believe each post and comment should stand on its own merit.