The benefits of a shop in my case:
1. Less weather dependent. You still get cancellations, but not as dramatic. It is amazing what weather people will ask you to wax their car in.
2. Credibility. Certain customers like the sense of location you offer.
3. Visibility. Depending on the location, the shop offers more advertising than you could pay for out of a mobile ad budget.
4. Good working conditions. A shop is (almost) always warm, has a bathroom, has hot water on demand, has an even floor, has a place to do paper work or sit down for lunch.
5. Retail opportunity. Having some products allows an opportunity to teach customers about detailing and make some retail sales.
6. Product proliferation. Space has allowed me to become an expert on problem solving and to always have a chemical or solution nearby to tackle weird and wildly variant issues.
7. Real estate development. Shop-based business will allow you to look at owning down the road. No matter what happens to the industry, have your own place would give more value to your business.
8. Overflow work. I make my shop more efficient by taking on lower priority dealer cars that can sit in my shop for a few days until they get done. If retail comes in, they get set aside; if a cancellation comes, there is always something to do.
Disadvantages:
1. Low season. In many places there is a lull in the season, and the cost of the shop maintenance in this season is quite high. The extra rent and insurance and phone and alarm and (potentiallly) staff and heat and lights add up.
2. Cost. As I mention above, there are great costs.
3. Customer service time. Gaining visibility means maintaining more customers which can be costly. You will probably charge more than many can or will pay, so you have to weed them out. In spring, I mostly do technical work and deal with customers. But hiring extra shoulder season help is okay (see below).
4. Overhead and casflow. You may be able to handle the cost of a business, but there are cashflow issues--more dealer work, increased staff, more accounts, more products etc.
5. Economic dependency. No matter what happens to the economy now, you can sell your van & truck and take a job at Wal-mart changing oil and do a little on the side. You get into the real estate, and you are committed to rent/lease or to the real estate market.
Just some thoughts.