You haven’t mentioned anything about your preferences, and these two hikes are about as different as it gets!
Manoa is beautiful - almost always overcast, but the view makes up for it. Easy hike - just under two miles IIRC. Easy to get to, they’ve recently added gravel to the path, although there are complaints about the gravel they’ve added making the paths harder to walk. Beautiful bamboo forest on a side trail. Lots of Jurrassic Park (and Lost) scenes filmed here.
Waimano falls is much more difficult - more difficult terrain, not well marked that I remember. Only been once, so don’t really remember many details. (Manoa is a few blocks from my sisters house, so that’s just walking around the neighborhood for me).
Rainfall’s been low, so the falls may not be very impressive. Bring bug spray!