I had the Nexus 5 for about a year and a half - it was my first introduction to Android (coming from BlackBerry) and I LOVED it! It worked very well and it was easy to navigate. It was a great low-cost phone that still did everything its competitors (at its time of release) could. I highly recommend it. However, I have heard that the Nexus 5 is not being manufactured any more at this point (you should check this out though, I mean, I *see* them being sold still; but from what I've heard, Google is not making more?); so that might be a reason to go with the 6. Also, if you're the kind of person who plays a lot of games or watches a lot of movies/TV on your phone, I would recommend the 6. I watched *a lot* of Netflix on my Nexus 5 and it worked perfectly; but having a bigger and better display would definitely enhance your experience.
All in all, the 5 is a fabulous device and it would be a great steal if you could get it for $150!