They're all good machines. You can make perfect miters with any of them, so long as you keep them in good condition. The differences among the brands and models are the various features affecting speed, convenience, and durability.
You are right to gather information and make an informed decision. Asking for recommendations here is a good idea, but understand that most of us have direct experience with only one or two machines, and we generally tend to prefer the machines we use daily. While we may be fully qualified to recommend the machines we have found to be suitable for our own applications, that doesn't necessarily mean any particular model would be right for your situation.
I suggest you get acquainted with what's available, at least the major brands, and decide what features are most appropriate for your business -- not only now, but over the next 5 to 10 years. As you noted, this is a long-term investment, so it makes sense to buy the machine best suited to your long-term needs. Generally, the machines with time-saving features offer a fast return on investment.
I'm a Fletcher/Pilm 5700 guy. Several years ago I sold my perfectly-functioning ITW/AMP/Putnam/alphamacchine VN-42 and bought my Fletcher 5700. It has several features I like better than the VN-42, VN-2+1, and Pistorius VN-J (manual) machines I'd used previously, but mainly I bought it for the double vertical clamp assembly. These days the 5700 sells between $2500 and $3000, depending on the accessories you want.
The double vertical clamp is a huge advantage over most other machines, because it enables you to hold even the widest, most radical-profile mouldings firmly throughout the joining process. Some other models are available with similar clamp designs, but they are mostly the high-end production machines with computers, selling in the $6,000 to $10,000 range.
I like this machine so much that Fletcher pays me to demonstrate them at trade shows, and write articles about the whole line of Italian-made Fletcher/Pilm machines.
Pneumatic machines wear from the inside out, so there would probably be no symptoms of wear until the leaking starts. If your present Cassese machine is less than 20 years old and needs major repairs or replacement, you probably have dirty, wet, or unlubricated supply air.
Whatever pneumatic underpinner you buy, make sure the supply air is conditioned at the machine. Use an air filter/lubricator to clean, dry, and lubricate the air. The Fletcher/Pilm machines have holes drilled & tapped on the back of each machine, to accept that optional accessory, and I guess other brands do, as well.
It is not enough to have a filter on the compressor across the room, because just a few feet of air hose or pipe could condense enough moisture and produce enough particulate debris to wear the seals on the pneumatic cylinders.
