For a true all around vehicle (meaning something that will be used in all types of conditions...mud, rocks, snow, dirt, etc.) a true 4-speed case is worth every penny in my mind. You will find you will use all the gears in the t-case all the time. For one dimensionally built rigs (meaning a dedicated rock rig, or dedicated mud rig, etc.) a single case with a single gear ratio to get your desired overall crawl ratio where you want it is totally fine, but for rigs that see lots of different terrain, 4 speed all the way IMO.
But don't just settle for Atlas. Atlas makes awesome stuff, no doubt, but there are other great companies who make 4-speed cases that are just as good, such as ORD's magnum or NWF's Eco/Black/Titan boxes. Or shoot, if you want to get a doubler even cheaper, then try and find a 203/205 someone's trying to sell off (the only thing that kinda sucks about the classic 203/205 doubler is it's really only a 3 speed case, since the 203 and 205 have virtually the same gear ratios, that means you can really only choose from 1:1, 2:1, and 4:1...that's where going with a reduction style box such as the Magnum or NWF's boxes give you more gear options and make it a true 4-speed...similar to the atlas 4 speed...but no doubt the classic 203/205 is still a great option for lots of rigs). Just something to think about
One other thing to mention is length, an Atlas will be much shorter than a 203/205 doubler.