I think with modern ammunition the choice comes down to personal preference. The diffrent pros and cons for defensive shooting for both calibers come down to nit picky little things and both calibers are good defensive rounds.
They have a diffrent recoil. I don't like 9mm recoil. I prefer .40 S&W. I don't knock those who prefer 9mm. They get higher capacity.
Were I you, my consideration would be ammunition. You already have a 9mm and the ammo to go with it. What's the price and ease of aquiring .40 s&w where you are? If your planning on getting a .45 as well, that puts you up to an ammo cache of 3 calibers and three diffrent calibers to buy for practice.
I can't answer if I think your should or should not. I can only tell you what I would take into consideration when making that decision.
Personally, if I knew I could get ahold of plenty of ammo for all three, I would.