Although they were clearly related they existed for a long time, and the longer they existed the more they diverged. Their close likenesses are probably an illusion created by the evidence that remains and the original interpretation of them all as reptiles.
If we could show up at different times in the Mesozoic we'd be confronted with a vast array of creatures and find ourselves classifying them into far more groups than we can see at this distance, including creatures that were never fossilized or at least never found.
I'd bet that about half of them were more reptilian and half were more birdlike with some overlapping creatures. Some may be total surprises to us, things we never found and could never guess at having existed.
So, I doubt you can say that the whole group we include as dinosaurs were warm or cold blooded, but contained a spectrum of different creatures.