Help me decide which T-mobile plan I should get?

[Raquel]

Okay, so I want to sign up for t-mobile and get an iPhone 5s.
All plans come with unlimited text, talk, & web.

Apps I use all the time. vvvv
Instagram I post like 2-3 photos a week
Facebook, I rarely ever post pictures there just message, read, etc.
Youtube, I only watch videos when I'm really bored,
Kik, I use alot to message.
Snapchat, Use it moderately.
Games - eh, 25-30 minutes max a week
Email - Never send, recieve about 50 a month.

& most of the time I'm on wifi.

There's a choice between;
500MB - $50
2.5GB - $60
Unlimited - $70
+ $26/24months for the device.

This is my first phone and an extra late birthday gift cx.
There aren't overage fees but when you go over its reaaaally slow internet
Which data plan is the best for me?

And in addition,
If you're on home wifi or any wifi like school, mcdonald's, starbucks, etc, does it still use up your data?

Thank you in advance!