It is very hard to advise people on picking chairs without knowing a lot about their exact needs....
That said, if you like your current S646, then I'd be very inclined towards it as something you are familiar with... I wouldn't even consider a different chair unless I had done some pretty extensive test driving of a loaner to see how it handled doing the things you do with your existing chair - does it drive the way you want, and so on. I forget offhand if the F-3 is RWD or not, but I'd be extra insistent on doing serious testing if it isn't.
These days the PT types and the manufacturers in the US are very into pushing mid and front-wheel drive chairs instead of rear wheel, mostly because it's "fashionable" not because they work better, or let you do more... I'm told the RWD chairs are still more popular in other countries.
In terms of the design, it is true that the 646 is an old design, especially when considering the previous, only slightly different 626 models. However it is also the case that the chairs are fairly easy for users to work on and modify to make them what they want.... All their parts are pretty standard, and there isn't anything that makes them gratuitously difficult to work on. While the latest "Gee Whiz" gear can be cool, there is also something to be said for having a solid, well tested 'tank' that is going to keep on running after the Gee Whiz has taken a whiz
Permobile generally has a reputation of being very nice when they are working, but can be a nightmare to work on if something breaks, and they do a lot of very slick things that are cool when they work, but make repairs harder and more expensive. They have a lot of proprietary stuff in their electronics that makes it harder to service because the tools are harder to get (They aren't as bad as Pride, but almost)
I haven't done a serious count, but it seems like Pride / Quantum is the number one 'horror story' brand, but Permobile is pretty close behind them. OTOH we don't seem to see a lot of Sunrise / Quickie complaints... Obviously we don't hear from the happy users, but...
ex-Gooserider