No shade to any of the providers advertising here, but you should always purchase lifetime based on user feedback. No point buying lifetime just because the admin themselves of a provider replies and tells you they're the best.
If I were to choose lifetime (which I don't really think is a smart idea) then I would go for something established and trusted that don't discontinue products after banwaves or low playerbase.
I would suggest SyntheticSkill.
But again, lifetime is not always the smartest option in games like this. CS2 for example make sense since that shit refuses to die, but other games.. not really smart unless you really believe you'll be playing it for a year+
If you find a good deal though, then yeah go for it :D