I strongly believe that the bypass, direct-wire type is the only way to go. Why pay to keep the ballasts warm? It's similar to people who replace automotive turn-signal bulbs with LEDs, and then add load resistors to load the existing thermal flasher, instead of installing an LED-compatible one. Why waste the energy you're trying to save?
I recently had a few customer-supplied LED tubes burn out, along with a ballast or two, when mixed types were tried in various combinations. There was no warning on the tubes about mixing, and the customer did the un-boxing before we were called. We ended up installing new ballasts and fluorescent tubes, at his request, and cost.