The colors of the white gold beneath the rhodium plating may not match. White gold is not truly white and colorless and some white gold has a degree more color than some others. It depends on the alloy. Common sense would say that 18kt is likely to be a bit more yellow than 14kt because more yellow gold is in 18kt than 14kt. Strangely, it is not always that way. Some 14kt is very white and some 18kt is very white.
In terms of wear and durability, the amount of gold in a ring has less to do with these two issues than than whether the ring was cast, hand fabricated or die struck. The method of manufacture controls the density of the metal and denser metal, with less air trapped in it, is a more durable product. Hand fabricated and die struck win out over casting as a general rule.