I'm on the fence on who I should pick up next between Darius and Lee Sin. On one side Lee Sin is an amazing jungler and has very high mobility and iku. On the other hand, Darius has dunks for days and that's really funny to me. I haven't gotten a chance to play either so if anyone with experience playing these characters could give advice towards who I could get I would appreciate it.

If you want a challenge, get Lee Sin. If you want an easy champion get Darius. Both are good and fun to play, but Lee takes a lot more practice.

I say you shouldn't hurry in buying a new champion. Provided you are level 30 already and have a minimum experience of say... 1000 matches in total, you should start picking your favorite position. Then focus on that position more, but learn others too, because it'll help you in soloque. Then buy champions according to that position. If you already have some, learn those first, buy the necessary runes, buy at least 5-7 rune pages and only then start getting new champions.

Buying new champions without mastering at least 2-3 is pointless. You'll never improve.