Stihl describes their saws (in their print catalog) in three categories: Occasional Use, Mid-Range, and Professional.
The "Pro" saws are: (MS) 260/260 Pro, 361, 440, 441, 460/460R, 650, 660, 880, plus the 192/192T and 200/200T Arborist saws.
As compared to the mid-range and occasional use models, these upper-tier saws generally have a higher power:weight ratio. They feature things like adjustable bar/chain oilers, more effective anti-vibration systems, compression release valves, magnesium cases & covers, and optional full-wrap or heated handlebars. They are (generally-speaking) easier to perform routine maintenance on, and to overhaul when neccesary.
Those features are what makes them popular... if the saws are good enough for and accepted by the pros, they are plenty good enough for devoted part-time users & enthusiasts.
As to what they are worth in 'good used condition'? I haven't been keeping too close an eye on this, but seems like they sell for anywhere from 50% of new cost and up. This applies to all their saws, not just the "pro" models. Not to say there won't be bargains to be found, but it does appear to be a seller's market.
