Hi Jaydee,
Different types of innersprings have many different features including the number of turns in each spring, the coil gauge, the diameter of the springs, the shape and design of the coils, and the number of coils that all combine together to produce the response of that particular innerspring. The most meaningful spec for an innerspring is the weight of the steel that is used and this isn’t a spec that is disclosed by manufacturers. You can read a little more about innersprings in this article and post #10 here. Having said that … I would think that the Symetrics innerspring has more steel than the Verticoil and has better overall performance so it would be a more costly and higher quality innerspring.
Warranties only cover defects in a mattress and for the most part defects will show up early in the life of a mattress. Foam softening or the loss of comfort and support that goes with it that doesn’t also have actual visible impressions that are deeper than the warranty exclusion aren’t covered by a warranty even though they are the main reason a mattress needs to be replaced. While warranties don’t reflect the useful life of a mattress … it is true that a manufacturer will usually put a longer warranty on their higher cost or quality mattresses but this is as much about marketing than it is about the actual usefulness of the longer warranty itself. You can read more about warranties in post #174 here.
On the other side of the coin … warranties with smaller manufacturers like Jamestown are also more valuable because they tend to give their customers more credit in the grey areas of warranty coverage while the larger manufacturers are more focused on denying warranty claims or making their customers jump through hoops to have a warranty claim approved.
The durability of a foam encasement depends on the quality/density of the foam used and on how well it’s glued together and to the springs. Using foam for edge support is generally less durable and less costly than using steel because the foam replaces a couple of rows of coils and steel is more costly than foam. Either one can work well for those who sleep on the very edge of their mattress or for people that sit on the edge of their mattress (as long as the foam is high quality because lower quality polyfoam will soften and sag more easily than steel and can become detached from the innerspring) but it’s not likely that either one would be quite as important for a child as it would for an adult.
In most cases the “value” of the mattresses that are produced by the same manufacturer is fairly consistent throughout their product line and higher priced mattress would generally have higher quality materials (the foam layers appear to be the same although they don’t mention the density of the foam) and I think that the one you purchased is a better mattress overall mainly because of the higher quality innerspring and I would have purchased the same one if I had the same choices you did.
The most important thing you did was buy your mattress from a local manufacturer that has better quality/value overall than the larger brands regardless of which of their mattresses you ended up choosing 
Phoenix