A usual, I think there isn’t one thing that does it all
If you’re playing on the beach in waves and just beyond, even small waves, get the a SUP because an inflatable kayak will take on water and tip you out (always seems worse being tipped out of a kayak than being tipped off a SUP), the SUP gives you more chance of enjoying the waves
If you want to cover distance over sea then probably the kayak, but get one as sturdy as budget allows, lighter inflatables don’t track well and get affected by wind and current more easily. Have you considered a sit on top kayak as a better option if you really want to explore coves. I bought a bic ouassou for a combination of playing with a bit of exploring
In both cases, wetsuits needed, if buying anything inflatable watch out for offshore winds it’s amazing how quickly you can be off over the horizon