This, like future color fixing above, should be a super slim tiebreaker when trying to make the decision
So, in short: while ramp and draw are both amazing, Id prioritize whichever helps you make your land drops
For example, if Thrasta and a 2/2 Bear Cub are attacking a planeswalker with seven loyalty counters on it (and are unblocked), Thrasta can assign 5 damage to the defending planeswalker and 2 damage to the defending player
Its cheap, the ability costs nothing, you have no timing restrictions, and you can even eat Treasure and random artifacts like Mycosynth Wellspring for extra value
If its regenerate ability required tapping itself, like a Crypt Sliver on an empty board, you wouldnt be able to activate it after attacking