Very simply, what coil resistance/configuration do you intend to run, and are you expecting big flavor, big vapor, or something else? (Extra points for mathematical justification.)
TBH, a little bit of everything.
Initially, I'll likely be building lower Ω coils than my sigelei 20W will let me. I'm at the limit (.67Ω) and I need more power to blast anything lower than that (I also use mechs, but as I venture deeper in to the world of sub-Ω builds, I want a good regulated MOD). Even if it weren't at the sigelei's bottom limit, anything lower wouldn't fire well because there wouldn't be enough voltage at 20W.
I'll also be auditioning it for a long trip I'll be going on this summer running Kayfuns. Is it small/light enough? Durable enough? Stealth enough (though I may have ruined that with ordering bright orange!). Is it reliable enough? Will it hold a Kayfun gracefully?