I'm looking for something mid-range that works well in summer/winter. Should I be leaning towards composite or vinyl?
You're probably going to get a few opinions on this based on individual experience. Is there a reason you excluded aluminum? I have installed composite, vinyl and aluminum sliding doors and all of them have preformed well to the best of my knowledge (no complaints). They all require little maintenance. I've replaced the rollers on my aluminum doors after 30 years and really appreciated the strength and durability of the aluminum doors. I've also noticed the track for aluminum doors are narrower than for the other types as are the frames so there's more glass to look out (and clean,lol). Because of the strength of aluminum, it can support a larger area so you can get by with fewer doors.