not going or unable to ?

[thomas615]

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

[Barb_D]

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

Do you understand the difference between the two sentences?

[thomas615]

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

[Barb_D]

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