I prefer Lemmy for:

• actually engaging with content (commenting/posting/voting) instead of simply consuming. By the time the API restrictions came around and the ads/bots started to dominate, it felt pointless to engage on Reddit any more.
• the positive parts of the federated and FOSS nature. Choose an instance, build your own, use or build any client you want to, federate or defederate whoever you want.

I prefer Reddit for:

• getting info/recommendations on things. The knowledge base is magnitudes larger than anything Lemmy can offer atm. Also, due to the centralized nature, it's so much easier to search for something on Reddit.

Lemmy's got some problems and I can't stand the interinstance drama, also, due to the decentralized nature, some instances can't keep up or the admins don't care any more, so whole communities can essentially be held hostage or simply die until a toolset to move a community from one instance to another (and propagate the change properly to the Fediverse) becomes available.