Definitely coffee .... every time I go to start a poker session in the afternoon .... one of the first things I do is prepare my coffeemaker .... this way every time there is a break of 5 minutes I serve a little cup of coffee ... and I feel totally relaxed ....!
I never drink coffee. I love tea on a cold or gloomy day while playing poker, but usually I just drink flavored water. When I need a pick-me-up for poker I'll make a green tea matcha smoothie.....Yummy! and it perks me up for focus but keeps me relaxed.
I don't usually eat or drink during poker, but water is probably the best choice if you do. Going by the options of only tea or coffee: it depends on which kind of tea and which kind of coffee - I've been known to have either. However, I consider myself more of a coffee person; so, I'll go with coffee of the two options