I drink both with nothing added except a tiny tiny pinch of salt in an espresso - they don't affect my blood sugar at all though some people say that the caffeine in coffee can affect them - the only way is to test and see, if you take milk try swapping for cream as it has less carbs
I drink both - with coffee I have a latte of a morning and tea the rest of the day. I have discovered though that having a cup of tea whilst eating my lunch or tea can send my readings up. Which I think is odd, particularly as a latte has no impact on my morning readings!