Answer to 18K comparison:
The difference between gold fill and vermeil is the thickness of the gold and the metal that is in the center. To be considered vermeil, the base item must be made of sterling silver and the gold covering it has to be no less than 10kt. To find out the legally acceptable definition of each of these products, I went to the FTC page Guides for the Jewelry, Precious Metals, and Pewter Industries.
And I quote:
"§ 23.5 Misuse of the word "vermeil."
(a) It is unfair or deceptive to represent, directly or by implication, that an industry product is "vermeil" if such mark or description misrepresents the product's true composition.
(b) An industry product may be described or marked as "vermeil" if it consists of a base of sterling silver coated or plated on all significant surfaces with gold, or gold alloy of not less than 10 karat fineness, that is of substantial thickness7 and a minimum thickness throughout equivalent to two and one half (2 1/2) microns (or approximately 100/1,000,000ths of an inch) of fine gold.
Note 1 to § 23.5: It is unfair or deceptive to use the term "vermeil" to describe a product in which the sterling silver has been covered with a base metal (such as nickel) plated with gold unless there is a disclosure that the sterling silver is covered with a base metal that is plated with gold."
As well as:
"(5) Use of the terms "Gold Filled," "Rolled Gold Plate," "Rolled Gold Plated," "Gold Overlay," or any abbreviation to describe all or part of an industry product unless such product or part contains a surface-plating of gold alloy applied by a mechanical process and of such thickness and extent of surface coverage that reasonable durability is assured, and unless the term is immediately preceded by a correct designation of the karat fineness of the alloy that is of at least equal conspicuousness as the term used."
Therefore, 18kt gold filled is definitely the better quality product.
Answer contributed by Dale "Cougar" Armstrong