I am admittedly biased here, but I’d go with Merlin. Both of my Wavers came off rate up despite me rolling on a couple of different rate ups before getting him, so the general pool factor is actually pretty big, on top of the fact that you can nab Waver with that tasty SSR ticket next year. Farming may be majority of the game, but I don’t think that necessarily mean it’s the most important, y’know? Ultimately we all play for fun and I don’t think anyone enjoys massive grinds with low drop rates. Having a native Merlin opens up a lot of options for enjoyable boss clearing builds and if that’s something you enjoy you should go that way. Merlin isn’t not useful for farming either, he’s just more CE dependent which makes him less helpful for garnering drop bonuses, especially in lottery season, but Merlin was the cornerstone of my farming teams for years before I landed my first Waver.
Waver is a much better pick in terms of flexibility but that flexibility is really only leveraged when it comes to scoring 3-turn clears with 5-6 event CEs. Merlin farming comps aren’t impossible to make provided you’re willing to leave off two event CEs at a time or have a couple of other tools - a strong Helena for instance, opens up a bunch of options for Merlin-based farming, and outside of lotteries most of us don’t get 5-6 event CEs anyway or don’t actually need 3T clears that badly.
It is also worth noting the Merlin banner will likely come after the 11-pull update, so your chances will increase slightly, but that’s a minor consideration.
Honestly, you can’t go wrong with either. It’s a matter of how much value you place on their respective roles and what you enjoy about the game. Merlin is better for careful optimisation and gimmick team building, Waver is better for high-speed clearing and resource gathering.