A quick primer on the types of lenses available in the focal lengths you are considering (with lots left out, or course, just the really salient points).
The most common 210mm lenses are Plasmat designs with f/5.6 maximum apertures. They are great performers and all take 67mm filters or thereabouts. Designations are Schneider Symmar (and successors, all with "Symmar" in the name), Rodenstock Sironar (et al.) and Nikkor and Fujinon W series. Some of us (me included) like more compact designs for the ~200mm focal length and look at lenses like the old Ektar 203mm, the Nikkor M series 200mm (rare now), the Schneider G-Clarons or Tessar design lenses like the Fujinon L series (not that much smaller than Plasmats) or even the slightly-wider Fujinon A series 180mm. These, being smaller, have smaller maximum apertures (~f/8), which makes ground-glass viewing a bit dimmer; a fine trade-off if you need small and lightweight. If you don't have an issue with the size of a 210mm Plasmat, they are always a good bet.
90mm lenses come in a few "families." The first are older 90mm lenses that just barely cover 4x5 like the Schneider Angulon lenses. More common, and much more useful if you need any movements at all are the wide-angle designs like the Schneider Super Angulon, Rodenstock Grandagon or the Nikkor and Fujinon SW series lenses. This last general category breaks down into two sub-families: the larger, heavier and brighter group, usually with maximum apertures of f/5.6 - f/4.5 and a smaller, lighter and dimmer group with maximum apertures around f/8 - f/6.8.
For example, there are Schneider Super Angulon 90mm lenses in f/5.6 with a large 82mm filter size and a Schneider Super Angulon f/8 lens (smaller) with a 67mm filter size. There are similar pairs in the Rodenstock Grandagon lenses (f/4.5 and f/6.8 respectively) and the Nikkor and Fujinon lines. The smaller versions of these lenses gnerally have smaller image circles, with the exception of the Nikkor 90mm f/8, which has an image circle as large as its larger cousin. Again, you need to weigh the parameters of size, coverage, brightness of viewing at maximum aperture, filter size, compactness, etc. to make an informed choice. Keep in mind that 90mm lenses often need recessed lens boards to get optimum movement capability on smaller folding field cameras and that the larger versions of these are a pretty tight fit in a recessed board. The camera(s) you are planning to use the lens on will help dictate whether you need a recessed board or not and then you can decide which size would work best in that scenario.
My personal direction has always been lightweight and compact. I do not own a 210mm Plasmat, rather some smaller lenses in that general focal length: I love my 203mm f/7.7 Ektar, but I've spent time adapting it to take 52mm filters instead of the push-on series filter adapter that was standard with it. I also have a Fujinon A 180mm (sweet and small) and a Fujinon L Tessar-design f/5.6, which is almost as large as a Plasmat, but takes smaller filters.
My 90mm lenses are both of the smaller wide-angle design: a Schneider 90mm Super Angulon f/8 that is really great, and a Nikkor 90mm SW f/8 that I got simply because I needed the bit larger image circle for architectural and city work. I keep this latter in my "city kit" along with my Fujinon A 180mm and the Fujinon L 210mm (surprising, but I often need them both in close quarters), a WF Ektar 135mm (again, for the larger image circle) and a 240mm Fujinon A (again, really small). My "field kit" has a 90mm Super Angulon f/8, a Plasmat 135mm f/5.6 lens (Nikkor or Schneider, I have a couple), the Ektar 203mm f/7.7 and a Nikkor M 300mm (again, fairly small with 52mm filters).
That's all just personal preference. Hopefully, though, I've given you some useful information along the way.
Best,
Doremus
