Both fine shops. Offshore is going through some transition including some phone troubles, but they (Kathy does their houseboat service scheduling these days) have been pretty responsive by email. JNB is literally a mom and pop shop. The J in their name is Jennifer Wells, who will usually answer the phone. the B is for Bill Wells, who runs the shop and does a lot of the work himself. JNB is a good choice for personal service and consistent personnel from year to year.
I wouldn't forget Northlake Boat Shop, the Aramark shop at Bullfrog. Some years they have great people in their shop.
I have found Aramark, JNB, and Offshore (old and new) to be pretty comparable for pricing. I have gone to Offshore more times than not because they have been great to work with when I want to do some work on my own boat while it's sitting in their yard. I try to show my gratitude for their being good to work with by leaving any space cleaner than when I found it. While Aramark and JNB are good people, they pretty much say no to my doing much more than doing some interior tidying on my houseboat. For something like drilling holes (over a tarp of course) to attach or replace boat accessories, they say forget it.
You can do well with any of these shops. I wouldn't wait too long in the year because all of them will get slammed starting in about April.
My small business is located in Hanksville. I am not set up to accommodate houseboats as of now but I do offer repairs for boats that can be trailered in and also offer open storage. Also offer vehicle repairs too. It is Rabbitbrush Repair & Storage and I am the newest shop in the area. Still getting started but very competitive pricing. I'm on Bing, Google, and Yelp. Please spread the word.