I'm trying to create 401k since my company matches 100% upto 3% of compensation and 50% of next 3%.

My questions is what to do between pre-tax and pro-tax(it's called Roth?).
I read a lot of threads but I still cannot get a clear understanding.

I'm in 15% income bracket.
expect to move up to higher bracket in few years. (I should add 'hopefully')

Could you please tell me pros and cons? maybe in real numbers?
Any advice will be appreciated deeply!

Thank you