I have been using a 6P for about a year now and I can say without any caveats that it is by far the best phone that I have ever owned. I also just bought my wife a Nexbit Robin when they were on sale at Amazon recently. Which so far from what I have seen, it is a very nice device. It seems very smooth and I honestly love the unusual design and UI. The camera is definitely a step down from her old phone but it is still good enough for what she uses it for. Also for her the battery doesn't quite make it through the day, but she uses her phones pretty heavily. For example she used to have a Galaxy S6 with a 3500mah battery case that wouldn't make it through the day either. So it's hard to beat up the robin too much based on her usage. Especially considering the internal battery alone is lasting about as long or even a little longer than the Samsung + external battery did.
However performance wise the Robin can't come close to matching the 6P. Yes the 6P is a little on the large side, but once you get used to it, it's not bad. Although honestly if there were one thing I could change about it, it would still be to make it smaller, because it is larger than I really like. However the benefits of the 6P are that it has a stellar camera, a very fast processor, decent battery life and it is going to get system updates much faster and for a longer time, than anything else out there.
Personally my opinion is that if the price is not a factor for any of these phones get the 6P, it's definitely worth the premium. However if you want to save a few dollars and don't mind a slightly slower but more quirky and interesting device the Robin is not a bad choice either. It is a very good midrange device just don't expect the performance of a flagship phone.
Honestly I haven't used a One+ phone so I cannot personally recommend or detract from it, but if I had to guess it would probably slot somewhere in between the 6P and Robin in terms of features specs and price and might be a decent compromise between the two.
Also since you are looking don't forget to look at the Nexus 5X. Performance wise it basically has the same specs as the Robin, but with the same camera sensor and system updates as the 6P, and in a smaller package. Honestly we had originally planned on getting my wife a 5X, but when the Robin went on sale it was just such a great deal it was hard to pass it up, plus she just really liked the styling of the Robin.