ABRSM or Trinity for grade eight piano?Watch
Maybe listen to the pieces for each exam board and see which ones you prefer?
Remember as well though to do ABRSM grade 8 you need grade 5 music theory, which I don’t think you do for trinity
I would do the one with pieces you prefer, as then you'll be more inclined to practice. If you decided you wanted to teach (privately or in a school), you would not need to have ABRSM grades. I'm actually pretty sure you don't need grade exams at all for teacher training, so I really wouldn't worry.
Also, as above, to do need to to grade 5 music theory to sit any grade 6+ practical exam with ABRSM.