These two are not comparable. Play what you fancy more.
We can only give extraneous reasons, which should really not determine your mood and purchasing decision, but might be of consideration.
Astral Chain will sell much less, so if you like Platinum Games and want them to continue making games, give them an extra copy.
There's going to be a shitload of games in the coming months, you can play and finish Astral Chain and still have time for the other games.
Conversely, there's going to be a shitload of games in the coming months, you can play and finish Astral Chain and still have time for the other games. By the time you're done with all of those games, Borderlands will be way past its prime, you may not want to buy it anymore (if you're tight on money this could also be a positive). Astral Chain will never stop being interesting, so in that case buy BL3 first.