All bars, all the time.
Personally I don't see any benefits for the loops, and a couple major drawbacks IMHO:
- Loops are less stable and require more physical force exerted by the user in order to gain a minimal amount of stability by pulling the loop taught.
- Loops feel dirtier and far harder to clean. Of course given that it's public transit it's not going to be fully sanitized or anything, but I feel like users can more easily take steps to protect themselves using bars (wiping the bar quickly before grabbing it, or shielding themselves by holding it through a piece of clothing).