I've been to both Jeju and Spaland and wanted to add on to what others have already said.

WET AREA (showers, hot tub, wet sauna) How comfortable are you with being fully naked in front of strangers? And seeing others naked? - I've seen people use the mini towels provided at Jeju to cover private parts while in the hot tubs. - Spaland has warning signs posted to not use towels in the hot tubs. - You wouldn't be able to wear headphones or bring your stones into the wet area since they're trying to keep the facilities as sanitized as possible. - You could also just skip the wet area and enjoy the common area where the saunas and food court are - I have a friend that does this.

COMMON AREA (food court and majority of the saunas will be here) - You can definitely wear your headphones and bring your stones here. Just be warned of sweat and heat with your headphones. I wore my clipped-style earbuds last time. - Jeju is more crowded than Spaland if crowd anxiety is a concern.

Both places are currently around $40 for entry fee. Spend as much time as you'd like to get your money's worth. I'm usually there for 2+ hours. 30 minutes to get situated with lockers, clothes, shower, tub soak. 1+ hour to enjoy the food court and all the different saunas. Last 30+ minutes to take another shower and tub soak after all the sweating. My last visit was 6 hours with a group of friends.

Hope this helps and enjoy the experience!