Hello from France!
Glassine or PrintFile(TM)?
Coming late to the discussion, actually I use both.
I had been using glassine sleeves in 135 and 120 for a long time (French brand Panodia(TM), still in business), and when I visited California in 1990, I bought some PrintFile(TM) polyethylene sheets. I had never heard of this brand before; nowadays they are well distributed in the Old World.
The only (minor!) problem I had at the time was: those sheets were punched on the edge for the traditional US-type 3-ring binder, so I had to re-punch them at home for the European metric 8-8-8-cm type of ring binder.
This problem no longer exists with current PrintFile(TM) products, on the edge, as far as I have seen
on their web site, they have all kinds of holes required for many kinds of ring binders.
Actually I find glassine sheets easier to load and unload than PrintFile(TM) polyethylene sheets.
Hence I continue to use glassine sheets for B&W or colour negatives, and I use PrintFile(TM) mostly for slides.
However, glassine paper is very sensitive to water drops! I did not invent the expression "wet darkroom" (we do not say that in French

), but those who bring their glassine sleeves in the darkroom know what I mean!
As far as the 5x4" format
[and may be larger formats for me in the future] is concerned, so far I have no LF colour slide in my archives, so I use glassine sheets bought from Macodirect on the Continent (German for glassine is Pergamin). 4x5" pockets work also for the smaller 9x12 cm format.
Regarding Secol(TM) products, this raises the painful question of how long our images on film will survive us.
Secol(TM) supplies museums: there, all kinds of documents have to be preserved for future generations: time scales are centuries ... and how documents are preserved is of the utmost importance, hence the choice of trusted plastics like polyester or non-acid papers. At a nominal cost ...