Both of these manures can be an excellent addition to your beds, Cheryl. Potency depends a great deal on what kind of diet the animal is fed. A rich diet heavy in alfalfa forage and grains can burn plants and roots if used in excess, so these manures should be used with caution until you become accustomed to their performance. Additionally, the same can be said for urea-soaked manures; a little bit can go a long way.
I've used both, but I haven't determined which is best. My horse is older and doesn't work, so she's fed timothy forage with just a handful of grain-based feed. I've found that I can safely use a great deal of this manure in my beds since it's fairly lean, but I do make certain that the urea is mixed in well before application.
Composting either manure first can ensure safety, but a lean-diet type manure will leach out a lot of its helpful properties if composted too long. Depending on diet, horse manure can have more weed seeds than rabbit, so if this is of special concern for you, opt for rabbit, or hot compost your horse manure to kill some of these seeds before use.
