I like this one because you can get it with sufficient pressure to work well with a "dry tool" (like the SS Upholstery Pro), where some of the other "dedicated upholstery machines" (KR 204/US Products Ultimate) and spotting machines have too little pressure and vacuum to work well with such tools. Plus that extra volume, vacuum and heat clean better than those units will clean.
Any good portable can give you the heat/pressure/vac to make one of those tools work well, but most are too bulky or heavy.
The Sphere gives you all of that, plus they are well made, durable, and readily available from most distributors for local service.