The generators and transfer switches aren't going to be different between dealers (same model number).
It all comes down to the following items.
I've seen some pictures of installs on other dealers websites that show a 22kW air cooled unit being fed by propane. The problem here is that using two 120gal bottles is ok for an 8kW generator, but there is not enough vaporization in colder temps for anything larger. The available BTU for that tank setup at 20F is 338,400 and that particular generator uses 335,000 at full load. You may say that the math works out ok, and it does. Untill..... we get a cold snap during a blizzard which is when you want your heat to work right? Guess what, at 0F you available BTU for that same tank setup is only 169,200. So, you thought that by hiring this particular company they would do the best job. Just because they buy more from Generac to maintain a certain status doesn't mean you get more. In this case, the customer will have to find out the hard way. Not only will the generator probably not work, but it has a maximum runtime of 40hrs at full load.
Some will try and sell you something that is not going to work, just to make a sale. The load calculations that are required for essential circuits, managed whole house and whole house have to be performed and followed. There have been a few times where I declined an estimate for an Air-Cooled unit since the load calculated was too big for the genset. When others gave a price for that same Air-Cooled unit and were ready to install. A larger 25Kw+ Liquid Cooled unit would of been required. The generator must be sized (by doing a load calculation which INCLUDES the square footage of the home) for all loads connected to it for automatic operation.
There are companies out there that sub contract the electrical and gas portion of the install. Why would you hire anyone but a Master electrician to install a generator? Its primary function is to produce power for your home. These days everyone and their brother is installing these. Especially Air Conditioning companies that allready have maintenance agreements for servicing their duct/ductless systems. Having different sub-contractors on a job leads to inconsistency with installation and scheduling conflicts.
Some will spread out some stone and drop your generator without regards to the installation requirements (I've seen many that were just dropped in the dirt). Or they will use 2"x4" studs or decking to frame in your generator, like that's going to last..... What's even worse is if they ignore the 5' minimum distance from windows, doors, vents, etc... which is to avoid CO from entering your home. Same dealers site shows a generator less than 5' from a bay window....
Using the incorrect wiring method in the installation is possible. People may use cables that aren't meant for wet location feeding the genset or mixing cables with incorrect voltage and temperature ratings .They may use Aluminum instead of Copper wiring (this is a preference and not a violation) since it's cheaper.
Some electrical companies use unlicensed persons on the installation and therefore underbid (mainly the electricians that don't have the state required 1:1 ratio of licensed to apprentice).
Master electricians aren't special and have to abide by the same 1:1 ratio with regards to Apprentices as Journeyman electricians do. You may hear from some that hey can hire as many helpers as they want, which is false. The only benefit is that Masters can hire as many licensed electricians as they want, whereas Journeyman are limited to one Apprentice. If you see an odd number of people wiring your generator make sure and ask for licenses. They must have it with them if they are working.
INSURANCE (Liability & Workers Comp)
You may also have illegal immigrant workers that have no workman's comp! So, what happens when they get hurt on your property? When in doubt, ask for a certificate of liability and workers compensation insurance. Most towns require a minimum amount of liability insurance for them to pull a permit. So, if an inspection isn't done then they might not have pulled a permit due to having no insurance.
There must be an electrical permit and inspection done for the installation (any electrical work done in MA). It is common practice for some to get their gas inspection, but no electrical. This is done to cut cost by cutting corners and not following the electrical code. This permit is required by MA law and must be done. The inspection is crucial since there may be many code violations. This may result in an unsafe installation that could cause fire, electrocution or explosion.
Just a note:
Those dealers listed as "Elite" or "Premier" have to make a certain minimum purchase yearly from Generac. This puts them at the top of the dealer locator.