With my limited experience, consider width (cover rear tire tracks), weight (heavier is better) and service (parts availability). Another good source for information and pricing is Tractor Ground Engaging Equipment . I can recommend the Ansung (Terraforce ) My YJR074 ids built like a tank.
The other thing is try and rent the unit your considering so you can be sure it is a good match for your unit. Sometimes you can go bigger and wider than the HP recommendations say depending on the soil type and your tractor. I actually went to a larger size after trying a smaller recommended unit out.
You can check out the 48" King Kutter XB series tiller on our website. They're made for subcompact tractors exactly like yours. The 48" Phoenix tiller is another great choice. We offer free shipping to a business or supporting freight terminal within 1,000 miles of Newton, NC and we also have product videos on our website for each of those tillers.
Wonder if there is a rule of thumb on hp for size of tiller? Seems that with a bush hog people say 5 hp per foot is a good rule, but what about other implements?