Tonilia and trading in thieves guild armor....

Do you have future opportunities to turn in thieves guild armor for better armor? If so, does it matter what items I turn in or should I do gloves, for example, over and over to get the highest pickpocketing?

So for example, I turned in my hood so I could get better prices. Do I have to keep turning in the hood each time to get the good armor, or is it not based on the stats of the item turned in?

You get one shot to trade in your armor. It only upgrades that one piece; And that's it.

Don't worry, further down the Thieves Guild Questline, you'll find some much better armor. Relax.