Depends on who you use, and what you need. If you have none of those characters, usually, Bennett is the best, he is better than a lot of 5 star supports and can also heal your team.
Fischl, is a good electro applicator with her Oz, and if you like electro reactions, I would recommend you take Fischl
If however, your choosing to buy one of these because you want their constellations, as early constellations, in my opinion, Bennetts C1 is very good, and Fischl's C2