The photos are not close up, I can't really tell if there is a difference in color.
You might want to double check with a jeweler to confirm if the rhodium plating with age differently on 14k vs 18k. I would hate to buy now matching and in 6 months see a noticeable difference.
For myself, if I was doing this to save money - wouldn't care if I stacked them next to each other.
Especially if I had a diamond like yours to stare at instead!
but aside from katatage being different , is it not also where the alloy(s) is from?
@soxfan, what would be the setting style? I find 14kt to be more on the lighter yellow side and 18kt more on the deep/richer yellow. Any dissimilarities between the rings that you stack may actually look pretty cool, because as @Rfisher indicated, there are differentces even between 18kt alloys. I wouldn't necessarily be worried about an extremely noticeable color difference, but if you really want 18kt and are only thinking about the money diff when compared to 14kt, then I would probably suggest just going for the one that you want, because it will be better to spend a little bit more money now, vs having to remake the setting in the future if you end up dissatisfied with the 14kt gold color.
I do like 18k yg because the color is so rich. But I think it looks great to mix up the different metals too. I have cvb bands in both 18k and 14k yg and I wear them together a lot. I really like the subtle difference and I think it makes each band stand out.
I'm one of those folks who don't like to mix my 14k yellow and 18k yellow gold. Just personal preference I guess. I can't think of a time that I didn't see the color difference in 14k and 18k yellow and mixing them in the same stack just bothers me. I'm okay wearing them on different hands though. Again, just personal preference. Why not go to a jewelry store and try it out for yourself. I know the golds are not all the same but i think 18k is almost always more golden and bright than 14k no matter the alloy, etc.
I have some 14k eternity bands that I mix in with 18kt gold and platinum rings. No problemo. For a plain gold stacker, the gold colors may differ, esp yellow gold. But white gold varies anyway based on the alloy used.
I did have a 14K yellow gold engagement ring and 18K yellow gold wedding band and I noticed a difference...so much so that I had the e-ring reset in a 18K comfort fit band to match..I like it much better now