Either is probably good. Phoenix uses a modified version of eclair afaik and Zeus uses LND. Phoenix has splicing support which is cool, spend a little less on fees when adding inbound. I prefer phoenix’s native UI and it feels faster. Phoenix supports background payments so you can close the app while someone pays your LN invoice

But really I think either is good. I can’t vouch for how Relai is doing their LN but it’ll probably either be custodial and not cool or perhaps with ondevice LDK and therefore infinitely more cool.