No real difference instead of that

Ты идешь куда-нибудь в следующую пятницу? - Sounds like the person who's being asked has already made up his mind about where they want to go on Friday. Of course the asker knows that the person they're talking to might have already decided where they will go on Friday
Ты собираешься куда-нибудь пойти в эту пятницу? - This one is like "Are you in proccess of deciding where to go on Friday". Although, that can mean the same as the first one.

Also, why use present tense for собираешься, because the word is literally translate as:

Are you preparing?

Why use present tense for идёшь? Because it's common to use a present tense for a scheduled activity. That's why I told you that the question makes you feel like the action is predecided

Я работаю завтра/ в воскресенье, понедельник etc

English speakers also do that with structures like that:

I'm leaving tomorrow

Even more, one English speaker told me long ago that when English speakers use "to be going to do something", that means that they're going to do that right now or in a very short time.

But in Russian собираться делать что-нибудь doesn't mean that you're going to get to it right now, just in a near future or even never (Like собирался, собирался, но так и не собрался)