If you are sure you need the space, the 128gb is worth it. If not, it's probably better to go with the 64gb. Somebody said the 128gb has the best resale value, which actually isn't true. If you look, say on a trade in site like Gazelle, you lose almost all your value for the extra space. They will pay you $370 for a flawless 64gb model, and only $375 for a flawless 128gb. Sure, you can try to find somebody private that really wants the extra space, but this shows that the market does NOT value the extra space on average.
So again, if you are trading in your phone every year like I do, you lose this extra $100 in value quickly. But again, if you are going to use it then sure go for it. Apple also clearly makes the most money on each higher model you buy as this memory costs them close to nothing. Hence why they keep the lowest model at 16gb, they want everybody to at least buy the middle option.