I teach Rabbit Agility and either the comfort harness or H harness will both work. I prefer the H harnesses (Lupine makes them in 2 sizes and they are listed in the cat section). The important thing is that the point where the lead connects does not pull on their neck, but is located on their back. The comfort harness will attach under the rabbit where the H harness will connect on top of the rabbit. Your choice will come down to personal preference. I find that connecting the harness above the rabbit is easiest for me and I don't need to deal with cleaning fur out of the velcro (H harness does not use velcro). I have a 12lb Californian and the larger size H harness fits her well and the size is much more adjustable than a comfort harness. A friend has a standard Rex, but Bear prefers to travel in her Radio Flyer wagon vs having to walk.