Nope - I don't regret it one bit
Essentially no difference in speed or features, other than apple pay (which I never used) and the larger screen. Battery still lasts me a whole day so no huge difference for me, at least not a 500$ difference.
That said, it also depends on preference. For me, the phone specs were too similar between the two devices. The bigger screen I liked, but the trade off for me is using a lighter phone with one hand more easily. I liked the "feel" of the iPhone 6 better however, it just curved into the hand, whereas the 5/5S model digs into the palm with extended use.
For others, they may prefer those advantages of the 6, and it may be worth the extra price.
Make no mistake, the 6 is an incredible phone so you are also giving something up too if you decide to sell it. It wasn't worth what I paid however, that was my rationale to selling it - it just didn't feel like that extra money was worth it over the 5S