They are closer than the price difference suggests
Both are unsaturated polyester resins dissolved in styrene, both cure at room temperature with MEKP and a cobalt accelerator, both are laid up by hand or sprayed, and both gel, exotherm and harden on a familiar schedule. A laminator moving from one to the other would not notice much.
The difference is in one ingredient. General-purpose resins are built with phthalic anhydride; isophthalic resins are built with isophthalic acid. Everything that follows comes from that substitution.
What the substitution actually changes
Phthalic anhydride has its two acid groups adjacent on the ring, in the ortho position. During manufacture it readily closes back into a ring, which terminates the growing chain. That caps the achievable molecular weight of a general-purpose polyester.
Isophthalic acid has its acid groups one position apart, in the meta arrangement, and cannot cyclise in the same way. The polyester can therefore be built longer. Higher molecular weight means more chain entanglement, which translates into better toughness, higher elongation before cracking and better resistance to crack propagation through the cured laminate.
The meta geometry also sterically shields the ester linkages. Because hydrolysis is the dominant long-term degradation route for polyester in wet service, making those linkages harder to reach is the single most valuable thing the substitution achieves.
Where the difference shows up, and where it does not
On a dry test coupon pulled on the day it cured, the two are not dramatically far apart. Headline tensile and flexural figures for a good general-purpose resin are respectable, and a buyer comparing data sheets alone may wonder what the premium is for.
The difference appears with time and water. After prolonged immersion, humidity cycling or chemical contact, an orthophthalic laminate loses properties faster, absorbs more water and is far more prone to blistering. An isophthalic laminate holds its condition longer. Weathering follows the same pattern over years of sunlight and rain.
So the comparison is not really about strength. It is about whether the part has to survive an environment, and for how long.
When general-purpose resin is the correct answer
It is worth stating this clearly, because over-specifying is as much an error as under-specifying. Dry indoor mouldings, decorative and non-structural parts, general FRP fabrication, moulds and plugs, furniture and panel lamination, and countless everyday components perform perfectly well in a general-purpose grade.
In those cases the isophthalic premium buys a property the part never calls on. The money is better spent on laminate quality, reinforcement or finishing.
When the step up is justified
Move to isophthalic when the laminate is immersed or repeatedly wetted, when it contacts dilute chemicals, salts or effluent, when it is outdoors for years, or when it is the contact layer of a tank, pipe, pool or marine part even if the structural laminate behind it is general-purpose.
A common and sensible hybrid is to use isophthalic only where it works: the gelcoat and skin coat of a hull, the corrosion barrier of a tank, the outer plies of an outdoor panel, with a general-purpose grade behind. This gets most of the durability benefit for a fraction of the cost increase.
What does not change with the step up
Isophthalic resin is not fire retardant, is not automatically approved for potable water, and does not make a poorly built laminate durable. Voids, dry fibre, incomplete wet-out and undercure will defeat a good resin comfortably.
Cure discipline matters more, not less. Chemical and water resistance depend on a fully developed crosslink network, so cure verification and, for corrosion service, post-curing are normal practice rather than optional extras.
How to decide in one question
Ask what the finished part will be in contact with, and for how long. If the honest answer is air, in a building, a general-purpose grade is almost certainly right. If the answer involves water, chemicals, weather or years, the isophthalic route deserves consideration.
Then bring the specifics — medium, concentration, temperature, exposure pattern, process and construction — to the enquiry, because those are what actually decide the grade rather than the product name.