Depends on location/plans:
If book hotel without breakfast, if i know there are enough possibilities to get one outside hotel, like in Silom area. Although i like breakfast buffet, i just eat too much.
Mo - Fr in Silom i go to that local restaurant/ food stall in Soi 6 between Tawan and Surawong for a plate of rice with lots of vegetables/meat to choose. After that i drink a coffee at Tiang & Tang.
If i am at a unknown to me place, it is more likely i book the room incl breakfast, especially if price is good.