Woodcraft and Rockler have carefully indexed catalogs with specific categories that make it easy to locate a specific type of bit. On Amazon, the catalog is not as carefully indexed but the shear number of bits they offer is staggering.
With router bits, the old adage 'you get what you pay for' truly applies. Two router bits might look almost exactly the same, but be entirely different in terms of quality. It all depends on how careful the manufacturer was in each step of the process.
The quality of the carbide used in the tips is determined by a variety of factors. Carbide tips are a composite material, which means that they take fine granules of carbide and mix them with a binder, which acts like a glue to hold the particles together. How fine the granules are is a quality factor, along with the quality of the binder material and how well the composite is 'cooked' to make the end product.
Even the best carbide tips are not much use if they are not properly sharpened. The finer the sharpening wheel that is used, the finer the grind, and better the cut. It takes longer to put a fine grind on bits, so here is where some manufacturers cut costs.
The carbide is attached to the bit shank with brazing which must be properly done, or the bit tip could fly off. Inspect any new bit you get, and if you see any gaps in the brazing, don't use it and return it. How well balanced the bit is will be determined by how carefully the manufacturer lines up all the parts and brazes them together.
You'll find cheaper bits, mostly manufactured in the Far East, where the overall quality of manufacture is lacking. The most obvious indication is a grind that is so rough you can feel a rough, almost serrated edge as you rub your finger nail along the cutting edge of the tool. Avoid these bits unless you just need a cheap bit for rough work and you don't care about the quality of cut.