All four of the things that you can upgrade with the spirit orb seem really handy to have. Early on the instant level up sounds really valuable and handy to have right now, but If I end up hitting the level cap before the game is done, It will have been a waste. Can anyone who has played more of the game than me suggest the best thing to do with the spirit orbs?
I would definitely agree that using a spirit orb to level up is a huge waste. Gaining levels is easy to grind whenever you like. It might be boring but it can be done. New hearts, more stamina and increased damage are not so easy to obtain.
Pick whichever one is causing you most trouble at the moment. The game is fairly easy. I think having 5 hearts is enough to get you through most of the hazards. So long as you have a healthy supply of meat or food that restores health, you should never have much problem getting killed. You will regularly need to eat during combat anyway so you will be killing two birds.
Early on in the game, focus on damage. It will save you lots of time having to hit stuff. You will get health from some temples. If you are not getting killed anymore, go for some more stamina. If you have more than enough stamina, go for more damage.