I definitely upgraded hearts first, but someone else might have a different point of view. also, you'll find later that you need to collect items/ingredients for NPC's to craft into your high level equipment. 

Here's a cool list of tips I found on Kotaku. I wish Minecraft let you mark the map like you can in Breath of the Wild. And add tags to the marks you make. It would be so helpful. 

Thanks. This is useful.

As far as I know your horse will not fast travel with you. You have to go back to where you left the horse and ride it to your destination. I remember that you can have up to 5 horses (! think). So you can leave them parked in different places. Once I got Epona I stopped riding the other horses.

That's what I thought. I'm having trouble taming wild horses outside of the starter area so I'm back to walking a lot again. I wish the game would at least explain this.

Stamina, at least one full circle more has helped me out a lot so far.

I'd go stamina personally, lets you get to a bunch of new areas easier. Like transmet said, get a full extra circle then start filling up some hearts.

Okay. I'm focusing on adding more stamina circle before hearts. I decided to run around the world climbing towers to fill up my map before pushing the story like I did with FF15. 

Good idea. Bare minimum you should have a second circle and about six or seven hearts total before you take a stab at a dungeon.