If you're happier with your 1520, then good luck to you. I can well believe your 1520 runs WM10 well, because I'm having no issues running WM10 on a Lumia 920.
However, having upgraded from a 930 to a 950XL, I have to say I could never go back, even though I have to agree with people who say WM10 isn't as well developed as WP8.1. The increase in screen size is so useful and the 930 really seems awkward and heavy now. Add the return of the much loved Glance screen and I'm willing to put up with WM10's clunkiness, given that in some ways WM10 is so much better.
I'm wondering if part of your experience is simply down to the reduced screen size. I know that when I pick up the 930, it does seem limited, even though it does almost everything the 950XL can do. Having that extra .7" of screen makes a lot of difference and you lost a whole inch.
I didn't go for the 950 because it seemed too much like a side-grade and frankly, I strongly dislike the shiny ring around the camera. In some respects my life would have been easier with the 950 - I could even use my old belt holster and both of my wireless chargers would work with it - but no, assuming Microsoft continue to develop WM10, the new devices are the way to go. I'm eager to see what HP are bringing to the table.