When talking about theatre we usually use the verb "идти" (it doesn't matter whether you are going on foot or by bus) unless you want to point out that you are going to use some kind of transport. Sentence a) corresponds to the English Future Simple used to express a spontaneous decision, a voluntary action or certainty. Sentence b) corresponds to the English Present Continuous used to express a planned activity.

if you're talking about agreements and planned future actions, as well as in English, you can use the present continuous (in Russian it's just present). Идти - it is used if for the movement you are using only your feet/ Ехать - This is used when you are using any form of transport - a bicycle, car, horse, carriage, etc. It does not depend on the distance. it just depends on the type of use of the vehicle, if it is

You can use both variants, because in most of situations your opponent know how you will get here (on foot or with help of transport) But "йдём" - there are no such word. True will be "идём"

Yes, both sentences are correct and equivalent. There are not mention of transport. But we can not say: "Сегодня вечером мы пойдем/идем в театр на машине" ! Я иду на автобусе/поезде/машине/самолете..... It is wrong! Always.

Hi, Francisco! 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.