Is the portability of the Switch a big plus for you? I use my Switch docked like 95% of the time since it's too big for my pockets and I'm not about to carry around a bag just for the Switch.
If you're like me then it just comes down to exclusives. Both consoles have some really good ones, but I absolutely loved Bloodborne. BotW was really good, but nowhere near Bloodborne. I haven't finished Odyssey, yet, but it's been fun so far. I let the hype get to me and bought Splatoon 2 even though I had the first one on Wii U and disliked it. Splatoon 2 ended up being my most regretted Switch purchase so far, Gonner being second most regretted. Mario Kart 8 was good, but after 3 starring mirror mode I haven't touched it. Maybe I'll try 200cc at some point. Never tried Arms, it doesn't look like it's for me.
PS4 has Persona, Project Diva, The Last of Us and Uncharted, all of which I really like. At the moment there are only two games I'm looking forward to; Shin Megami Tensei V and The Last of Us 2.
The Switch is light on rhythm games at the moment, which is one of my favorite genres. Tbh, because of this even my Vita gets more use than my Switch. That might change when Superbeat Sonic is released for the Switch, though since that's the game I've been most playing on my Vita.