Here is my take on this question and I am sure others will weigh in too. There is a lot of overlap within the sanders, but each one has its place. Some sanders that are ideal for finish work are capable of rougher stock prep with coarser grit paper -- and time. Sanders with a smaller orbit will take longer to hog off lots of material than a sander designed for more aggressive work. There are lots of special purpose sanders available to assist with linear sanding only (great for profiles, for example) and delta sanders ideally suited to reach corners. Pad size determines how wide a path you make with each pass -- some folks prefer 5 inch size and others prefer a larger 6 inch size. The smaller the pad, the longer it can take to smooth a surface and you have to pay attention to moving the sander to keep a surface flat -- it is easy to oversand in an area if you are not careful -- that is why I really like the RS2E sander for larger panels and for surfaces that I want to ensure stay flat.
Generally the orbital sanders (DTS, RTS, RS2E) excel at finish work. Great with higher grit papers; capable of more aggressive sanding with coarser grits but not as fast as an RO sander.
ETS 125 and the 150 /3 or /5 are good all around sanders -- capable of more aggressive sanding and fine finish work -- wide range of paper in different grits and pad density improve their versatility. Great for open field work and bigger areas.
Rotex Sanders -- very versatile machines -- good for hogging off lots of material quickly in orbit mode as well as finishing work in random orbit mode -- the 150 has the same orbit in RO mode as the ETS 150/5. Again, grits, pads, paper will determine how effective each one is for the task at hand. RO90 is a true multifunction tool with the Delta pad -- see below.
RAS -- truly an aggressive wood eating machine that is easy to tame -- think of it similar to a grinder -- excels at rough material removal, paint removal and carving, coping and sculpting wood.
LS130 -- Linear sanding -- great for sanding profiles like mouldings and hand rails, edges, etc... you can customize the pad to fit the profile at hand.
Delta Sanders - the DTS fits in here as does the RO90 with the delta pad and the DX93 -- again, getting into corners is what these excel at and into tight, narrow spaces. The RO90 is extremely versatile as it does the RO, agressive and delta modes in one machine. Smaller pad size makes it ideal for smaller surfaces like rail and stiles and smaller fields.
Air sanders -- not as popular, but for use when you have to sand for 8+ hours daily -- less parts to wear out, quiet sanders but need air.
Scot