Both sentences are correct. However, they are not 100% equivalent. The thing is the verbs with prefix "по-" (in 'a' sentence) literaly mean future action but with a shade of indefinity, uncertainty. In 'b' sentence the verbs are in Present tense, actually.....but we usully say so (I mean we speak about future using verbs in Present tense) when we are sure we're gonna do. So, if you wanna tell someone you're absolutely sure that tonight you go to the theatre, you have tickets on your hands, you should use 'b' sentence. If you not really sure, I mean you still could change your mind during the day, or the idea to go to the theatre appeared in you head just now, you'd probably use 'a' sentence. I hope it makes sence to you!
But, please, keep in your mind that the difference discribed above becomes obvious when you think hard. In everyday life using both sentences is appropriate and mutually exchangeable, so it doesn't really matter which sentence you use.