Most of the ardent flame abilities now have magicka and stamina morphs. So you can tailor them to whichever build you'd like to go with.

It really depends on whether you want to focus on using class abilities or weapons. If you want to focus on weapons, bow, 2H etc you should go stamina. If you want to focus on class abilities, a bigger magicka pool will allow you to use more of them. I tank with a magicka build. I still need some stamina to block, but all of my attacks are magicka based. When I want to dps, I use a destruction staff. It really depends on your play style and how you want to build your character. Both will work really well.

Keep in mind that if you want to go stamina, the medium armor passives will support that. The light armor passives support magicka builds. My strategy is to level both. I wear a combination of armors while leveling, so I have the option to switch later if I want to change my play style. You can always change your mind and reset your skill points to try something different in the future. By the time you reach V14, you will have 250+ skill points to play with. That's enough to use several different combinations of weapons and armor.

At level 20 you're just barely getting started. Don't feel like you have to lock yourself into a certain play style. It's easy to change you're mind later (just costs a little gold to reset). So, stay flexible and try different things until you find a play style that works for you.

There are only two stamina morphs in the Ardent Flame line, and they aren't very strong on their own, if you want to take all Ardent spells then you need to invest heavily into magicka.

Just depends on what your goals are. What your play style is. I tried out 2h and DW and they both yielded good dps numbers but it was just really hassle-some to have to share my dps pool with dodging and blocking. 2h was boring to me. Wrecking blow, LA, wrecking blow, LA, execute execute execute. Just blah. I prefer to keep my dunmer DK a pyromancer. Yes the LA nerfs were hard but with the monster sets coming in heavy and medium and the undaunted passives encouraging a mix of diff armors it's really not that bad with a healer or some defensive moves. & the numbers are also highest magicka dps output I've seen so that doesn't hurt.

Try burning breath and unstable flame then.

I had the same issue at first and after few tests, the best way forward (for both PVE and PVP) is to go full on Magika. Remember that magicka isn't just to cast more spells. It also directly determines the strength of the spells/DK abilities you cast. If you're a dunmer, you wanna go fire-mage. Your second ability can be either 1handed and shield (great for passives and for the active skill Invasion) or Restoration staff (depending on how you'll want to heal yourself). Put all your skillpoints in Magicka untill you reach the Cap. After then, put the rest either in health (if you feel you're not keeping up staying healed) or Stamina (for dodge rolls etc). In any case, put a couple of points in Provisioning and make yourself some food to boost either HP or Stam and you're OK.
The only tricky bit for this build at first is taking on multiple enemies without dying, since most DK abiulities don't have a huge range. But once you get to level 15 you should roll like a boss!