Lemmy has certainly already won me over reddit. Going back to centralized social media is something I will actively avoid if at all possible.
However I believe nostr is a theoretically better protocol than activitypub. Having your account/identity tied so strongly to a particular instance is undesirable. As soon as there is a reddit-like (or even forum-like) client for nostr which is relatively active/polished, I will switch. Nip 172 can't come soon enough.
In addition to being less popular / newer than activitypub, nostr is also full of bitcoin[^1] bros and twitter refugees (not my crowd). But frankly I think complaints about that are like the complaints that lemmy is a place for tankies a couple years ago when people's only exposure was to a (much smaller than today) lemmy.ml.
[^1]: monero support would be nice though...