18 inch chain looks better on you than 16 inch. Can you trade in current chain to offset some of the cost? I'd go for at least 14k white gold, so you can share the chain with the second pendant as well.
If you decide to set it on a ring, will you wear it daily or occasionally? I am concern about how the high profile ring will hold up on daily wear, can you wear it at work (if you are working that is, some places don't allow ring but are okay with necklace), and if you mind purple clash with some nail polish colors

Keep the square set with curve pointing outward if you plan to wear it occasionally.