not going or unable to ?

I will not be able to join you all in the trip to San Francisco

OR

I am not going to join you all in the trip to San Francisco


Are there any grammar errors in the two sentences? thank you

I would say ON the trip to San Francisco, and the word "all" is optional.

Do you understand the difference between the two sentences?

No. Could you please explain the difference between the two sentences. Thank you

I will not be able to = something is preventing me.
I am not going to = I simply am not, without giving a reason.