No, the West Marine epoxy costs twice as much because it is marine-grade quality. Plus you get a high-quality, genuine plastic pump with it.
Marine grade describes the willingness to spend twice as much due to the fear of sinking many miles from shore. They have perfected a magical formula, as secret and unique as the special sauce on a Big Mac, which resists salt water, ultraviolet rays, and common sense.
Ordinary manufacturers of products are incapable of fully understanding the marine environment and prefer to design fragile, weak, inferior products in mass quantities which will then unravel, fall apart and disintegrate in the presence of motion, salt water and sunlight.
This is a huge misconception that a lot of people have. Most of the manufacturers that I deal with don't take any markup at all because something is going into the marine world (heck we try to shave costs for marine use). There are often real and fundamental differences between a marine part and a standard part, but these differences are not often immediately aparent unless you know exacally what to look for.
One of the best examples is in light fixtures btw. Lowes sells tons of low cost brass light fixtures, so why should anyone buy one from West Marine for double (or more) the money? Simply because West requires their products to be admiralty brass which is an alloy specifically designed to resist de-zincification. It is harder to work with, has a more expensive metal mix, and is used in small quantities than 'common brass' that is about 40% zinc. For most applications having a high zinc content isn't a problem, but put that part on a boat in salt water, and you can almost watch it disolve.
Standard hardware store epoxy like the Devcon sold by Ace has a tensile strength of 1,500psi. Which is pretty strong. But compare it to West System with a tensile strength of 7,900psi, or roughly 5 times stronger. You may it need that strength in every application, but that isn't the same thing as the two are the same.
And those pumps are awsome. Until you have spend days cutting uncored epoxy out of a keel trunk with a chisel, because someone mis-measured the hardner, you can't know how nice it is to know that so long as they use the pumps they can't get it wrong.