Its really tough to say.
They kind of need each other. If you can go high, thats good but it means nothing if you cant get the volume of tetras needed to even have to go high. If you're fast, thats good but if you can't get high enough, you have to stop after only a few tetras.
If I could only choose one, fast seems better to me. At least once you hit max height you can defend or something.
Both would be the real way to go. I dont have video to back it up, but I do have the classic pic of us and 229 stacking. Both of us are fast and high cappers (utilizing the same shuttle and fourbar), as
this goal was stacked by 229 and 217 back and fourth