I have used all 3 products. Sunbrella (9oz) is a patented acrylic product of Glen Raven mills. (Glen Raven also makes Harbortime, Surelast, CoastGuard, SeaSprae which are different weights of acrylics) One has to be careful in that there are lighter materials, plus there are copies from China, which look the same as Sunbrella, but don't last. . Sunbrella is coated with a fluorocarbon 303 fabric guard to make it more waterproof. The Sunbrella should be cleaned each year (as per their instructions--easiest is to use the 303 cleaner, then new 303 fabric guard applied. I have had Sunbrella last over 10 years even without 303 protectant on sail covers.
Sunbrella PLUS is regular Sunbrella undercoated with a Polyurethane backing to make it more water and abrasion resistant. The underside is still the same color as the topside, but the clear Polyurethane gives it a clear, coated look.
SeaMark (11.5 oz) is WATERPROOFED Sunbrella achieved by coating standard Sunbrella on the the backside coated with Haartz textured marine grade vinyl. SeaMark offers the often desired look of Sunbrella with the added protection of a high strength vinyl coating underneath for the best protection in wet weather while remaining visually appealing.
There is also Sunbrella® Supreme. Basically it is a varient of Sunbrella PLUS with a flocking (short fibers) adhered to the underside to give the underside a decorative finish instead of just the clear coated look of Sunbrella PLUS.
SailRite has an excellent PDF on comparison of the various fabrics if you google "Acrylic fabric comparison Sailrite".
Also you want to use Tenara Thread (probably the best, but there are cheaper threads) for its UV resistance and longevity.
Our experience is that a Sunbrella Bimini is the best--it breaths, and does not have the condensation which a coated material has. It is also less susceptible to mildew. We have maintained waterproof material with the recommended cleaning and 303 applications over the life of the product.
As the coated materials age, there can be leakage around the seams. Of course this can be coated with various sealants.
We have an average of 64" of rain in Pensacola a year, but there is a high humid and warm atmosphere in the summer. We have been happy with the plain Sunbrella during our cruising of the PNW in a number of boats--but it is possible that with occasional snow load, and heavy cold rain you might prefer a coated fabric.