Foremost is a specialty insurer that some insurance companies (e.g., Farmers) use to insure motorcycles.
My experience is that if you are dealing with a widely recognized insurance company then there generally isn't much difference in claims handling. (Individuals have unique experiences good or bad; but, across the board there's not much difference between the large carriers.) AARP does not provide insurance but it does provide a "brand" to an insurance company; in this case, The Hartford.
There are a number of threads on ADV relating to insurance. Be aware that insurance is highly indvividualized. Location, age, driving history, etc. will play large roles. Your best bet is to ask local riders in your area. In addition, make sure you are comparing "apples to apples" (i.e., coverages and limits) when comparing rates.