Coming from a 1520, you may want to consider a 640XL if available at your end and if it suits your budget. It is much closer to the 1520 in terms of dimensions, battery life is good and it runs the latest version of the Insiders Build very smoothly - no resuming screens, no lag and no stutter. Sure there are some glitches, but that is more a function of W10M and not of the hardware. The rear camera is good the FFC is adequate for Skype etc. I have the Dual SIM and LTE version. To be very honest, the only weakness that I think the 640XL has is the 8GB of internal storage. In my opinion, it should have been a minimum of 16GB. But it is what it is. I use the MicroSD card. I have a 64GB card (some apps, music, videos and documents get stored on it) which I will upgrade to a 128GB card as and when needed. I dshould also add that the 640 XL has been - in my experience - a sturdy phone. I have dropped it twice from a height and nothing happened to it - not even a scratch on the device. I also notice that I use it more roughly than my 1520 (which is also running W10M ver .76). Lastly, I have to say that the 640XL is one of the most comfortable phones I have ever handled.
Also, all what I have mentioned above with reference to the 640XL applies to the 640 too - this is what I gather from reading a lot of user opinions. I think the camera on the 640 is a little weaker than that compared to the XL version, but that is about it.
I don't think you will go wrong picking either.