Polyester-based filler vs epoxy-based filler is not really a binary choice, all or nothing, one or the other. Both have their uses, and the reasons can be subtle.
Big picture: Epoxy is superior in terms of adhesion, tensile strength, and modulus. Polyester is fast, simple, and inexpensive, with (in very rough terms) 20~40 % lower mechanical values.
Considering only these mechanical factors, an epoxy filler becomes a better choice when the application will be subject to more stress. An example might be the leading edge of a surface subject to insect and rock impact, or bridging a gap subject to differences in thermal expansion.
A faired surface, with no notable stress, doesn't require high mechanical values. A polyester filler allows speed; it becomes possible to notice a small imperfection, apply fill, sand, and paint in the same afternoon. Polyester resin is also less expensive, as its viscosity and cure are adjusted with inexpensive styrene monomer, often as much as 30%. Speed and price are huge drivers in the auto body industry, where the application doesn't require high mechanical factors.
One practical factor can be application position. A simple epoxy/micro mix is not thixotropic. It will sag and run to some greater or lesser degree when applied to a vertical surface. It may not matter (for example, if the surface will be subjected to a lot of contour sanding later), or it may be quite inconvenient. All the pre-packaged commercial polyesters are largely thixotropic, and even it they were not, they cure so fast that it doesn't matter.
There are chemical factors. For example, polyester resin cannot be applied directly to foam, so it doesn't work for contouring fast forms in the home shop.
There's an old rule which says epoxy can be applied to a cured polyester/glass composite, but polyester should not be applied to a cured epoxy/glass composite. The polyester filler industry has pushed hard to develop fillers sold as "sticks to anything", so the rule may not be quite as ironclad as it once was. However, given the average aircraft builder has epoxy on hand in the shop, observing the rule has little downside.
Polyester shrinks during cure and later, which introduces internal stress, so eventual cracking was expected with earlier polyester fillers. We don't see so much of that with today's improved formulations, but shrinkage still exists.
Neither epoxy or polyester is particularly safe. MEKP catalyst is blinding. Epoxies are notorious for sensitizing users to the point of bad skin reactions. You must use care with either system.
Polyester fillers are heavier, as the typical bulk solids are talc and limestone. The primary bulk filler is most epoxy formulations is microballoons. It obviously makes no real difference to airframe weight if the quantities are small. However, be aware that a heavier filler does affect control surface balance. There is at least one flutter accident on record in which a significant quantity of filler moved a rudder CG aft.
To the OP's question...Polyester is probably fine on the dent. Epoxy/micro would be better for the seam between the fiber glass tip and the aluminum elevator. You could use either for either, and obviously some do with success.
Recent practical applications in my shop...Rans has builders cover bolt heads which protrude through the front of the tubular leading edge spar. The heads ultimately are under the fabric, thus the fill or a cover plate. Some filler also fairs the junction between the spar and the leading edge upper surface sheet metal. This is, hands down, a polyester filler job. Both wings, from start to done, took about 45 minutes.
Carbon turtledeck shell...one ply of 6K twill on each side of a honeycomb core. Although either polyester or epoxy filler would work, epoxy micro is much lighter in this quantity.
.