From pure powerlevel point of view - if you're not planning to spend money anytime soon, as your only 6*, I would say Maple is better. Serissa, on the other hand, is far superior once you team her with second or third rainbow.
Also, neither of them has bloom form, so you're taking some risk here.
From waifu point of view (only point of view that matters TBH) - pick one you like more.