So I got the codebook quest and after a lot of thinking I've decided to stay with the thieves guild. How do I choose one side over the other. Do I just not finish the quest for the eydis fire eye? And keep doing the Thieves guild quests?

Solltide told me to find out where my loyalties lie. Is there a way to tell her that I wan't to be in the TG or what? I'm confused.

I'd like to stay in both guilds but whatever I just need to figure out how I "choose" one side over the other?

Any recommendations on what I'm doing wrong here?

Thanks.

Also... in you guys' opinion if I choose the thieves guild over the fighters guild am I missing out on a lot of cool stuff? I wish I could just do both.

You can do both but you'll have to ignore the rest of the Balmora FG quests. Go to one of the other cities for more quests.