If you’re considering getting a Reefer or Max S-series you really should ignore the cost of them because the price really isn’t relevant to the choice. In the size you’re talking about, you can make a pretty amazing aquarium out of anything given a $4,000 budget.
The Max-S series of Red Sea aquariums are geared toward people who enjoy the hobby but don’t particularly want to get their hands dirty so to speak. The systems are all in one, have been designed to support mixed reefs and provide everything a healthy aquarium needs. This is a great choice for someone who wants to buy fish and coral and feed their aquarium.
The Reefer series offered by Red Sea is specifically geared towards enthusiasts. Red Sea offers this line for people who want a beautiful rimless tank and stand but don’t want to pay the premium of custom work from various builders in the hobby. They offer low iron, thick glass, and black silicon with inserts to protect seams. The tanks look and feel very well built and I wouldn’t worry about Reefers having seam failure.
If you want to be able to research the exact lighting or skimmer or return pump or plumbing manifold and reactors, the Reefer is the obvious choice. If you don’t want to have the freedom to design the aquarium filtration or lighting, then go with the Max-S because your life will be much easier and you know you’re buying a complete product.
The price difference between them isn’t really relevant because it depends on your equipment goals. If you want all top end equipment then yeah, your aquarium is going to end up costing more than the similarly sized Max series. However, in that scenario the comparison is meaningless because you’re comparing stock mass production to borderline elitism equipment choices.
I personally own a Max aquarium but I am in the process of ordering and setting up a reefer as well. You can’t go wrong with either, but from personal experience I would 100% recommend the Reefer because of the aesthetic of rimless as well as having a lot more freedom in choosing what components to use for your lighting and filtration.