Deckster
I will explain from my perspective. Of course I am biased towards the Deckster, so don't trust me.
The Deckster has been around for 10 years. It was designed by deck guys for deck guys. There are about 1000 of them out there. The first one ever made was still in service last fall when we serviced it. We own a prototype that we have used since 2002 in our shop.
Over that time, Decksters have had three different pumps (each one much better than before). The current pump is pretty much the same as the one on the RS rig. They saw a good thing and adapted it, which was smart of them. I was flattered by that.
The battery case on the Magnum is large, designed for a regular battery. We tried that at the very beginning. but it made the sprayer way too hard to maneuver. Too much weight. After a year of searching, we found the perfect battery. It is a gel cell, as small as a motorcycle battery. It holds a charge for two days between charges. The battery is a little pricey, but I think it is worth every penny. I can't remember exactly for sure, but i think there is about a 30 pound difference between the Deckster battery and a standard battery. That amount of weight is huge. BTW, we never updated the picture online (shame on me) but our battery box is half the size the picture shows.
The Deckster comes with two wands. One is plastic for shooting chems (cheap and durable) and the other is a really nice trigger gun with an adjustable thumbwheel in the handle so that you can cut down your flow (like for spraying spindles) even while you are spraying (without going back to the sprayer). Very sweet. The Deckster comes with a large brass cone-jet tip to make sealer spraying extra fast.
The Deckster comes on a foldable frame. That frame costs more, but that may not matter to everyone. The Deckster is powder-coated in a bright color to look like a nice, expensive tool. We considered the hand-truck approach at first, but I thought it was more important to have a real professional look to this sprayer.
The Deckster also features a clear-cover in-line filter. We plumb that filter in reverse so that any dirt shows on the outside of the screen and is easily visible. These kind of tweaks come from our experience as deck guys working in the field.
Please also keep in mind that we carry all parts in inventory to keep a Deckster in top shape and we also service them. That means that a customer can call us and get technical support and parts. We can look in our system and see everything about your sprayer. We also offer Deckster rebuilding classes frequently at RTs.
The Deckster costs more because of things beyond the pump (even though the pump is the best individual part). There is a $150 price difference, as you point out, but the battery alone comes pretty close to that amount. The extra items included in the Deckster tip the value scale way towards the Deckster IMHO.
For most customers, it comes down to a choice between getting a lower price or getting all the perfect tweaks. I like the extra tweaks and features, and I'm the boss. I get what I want. You should do the same.