What is worth spending on

I'm new, been playing about 6 weeks. My first recommendation is to do what you think will be fun. If your idea of fun is competitive success, then I definitely recommend the Phoenix route. I didn't go that way since I didn't understand all the game mechanics, and generally wasn't interested, but they have some glaring advantages early/mid game, namely protection regen, that for other teams is only available way late in the game (like zeta abilities).

This gives major advantages trying to clear Galactic War (unlocked at level 40 and requires completion of 12 consecutive battles) with a single team. Around level 60 if you focus on any other team it will be impossible with any other good arena team that can't revive, passively heal, or regen protection. Phoenix can do all three. Plus, their AI is pretty good on defense (squad arena) and as the guide states they can unlock two legendary characters and they have some ships. I pretty much only face Phoenix and Nightsisters in arena in the top 300.

If you're just not interested in them then go another way, because playing something you enjoy lore-wise is in my opinion more important.