Look at both a factory built California and conversions. Bilbos are very good.
Compare power of engines (conversions based on commercial vans can be a bit puny, I think the lowest Cali is 150PS), transmission, quality of cabinet work, materials (lots of laminated particle board is used for conversions) how it is cut, is assembled together and how securely it is fitted. There is some very shoddy work. How waste water is handled. The gas compartment of a California is very secure. My Bongo conversion was ropey in comparison. The simplicity of the power and control panel. Again the Zig unit on my Bongo was a pain. Much prefer the top loading fridge of a California, you can get to it when the bed is down and the contents don't roll all over the place. The storage of things like tables and chairs is pretty neat in the California compared to a conversion. Compare the top beds. It's very high quality in a California.
I looked at both, there is a huge choice of conversions. I bought a 18 month vehicle from VW in the end for a price that I didn't think was too bad compared to a conversion.
California's are not faultless. VW are not that good generally. I've had two failures of the electro-hydraulic roof. They also have an issue with galvanic corrosion of the roof where steel meets aluminium. It's not penetrating corrosion, but does bubble the paint which is unacceptable to owners of such an expensive vehicle. VW has a massive waiting list for repairs. My hasn't experienced this yet.
You might want to also look at the Mercedes Marco Polo.
Buy in Autumn/Winter, sell in Spring/early Summer.
Toilets - Finland.