I think that all three trees provide comparable bonuses to science. Similarly all trees provide bonuses for tourism. Therefore I would pick according to my situation.
Fealty favours wide, defensive, being a founder, WLTKD synergy and uhhh having a higher than average number of Pastures.
Statecraft favours active play, mostly tall with a strong Capital and decent food and convenient (for you) placement of city states.
edit: damn I forgot about chanceries and wires, I guess wide statecraft can be pretty good if you can secure those CS, it's just that the tree bonuses by themselves are the worst of the three in scaling with many cities instead of a lot of Pop
Artistry is the easiest tree to earn culture with and favours Golden Age synergy and GP generation (including Scientists)
I guess splitting the trees is an option.. feels a bit inefficient and indecisive I think. There is the specialist synergy and ehh I guess the Fealty growth-Statecraft Capital scaler. Adopting a whole tree helps fully realise and develop a specialised strategy though, at least for me.
I guess the Medieval trees are a bit weird in that regard and even put off many people that just picked up VP. I've seen many newbies just wanting to go for Tradition Progress instead of picking one of them