My Wife has a 1020 and I have a 1520 so if it helps I see the back-to-back differences in some of their features almost daily. (and while I've posted some of those in a number of threads here to be fair even I can't remember which threads so I'll try to recap quickly).
Camera differences do exist. Most of the time they are subtle but a few things stand out. The fact that the 1020 has a mechanical shutter (instead of doing it purely in software) does translate to less blur in action shots, and if the light levels are there, pictures will have more detail under close scrutiny. But unless you pulling up the pics in a desktop image editor (i.e. Photoshop) I honestly don't know if you would be able to tell the difference 90% of time. The one area where the 1520 shines is speed (both in focusing/locking on to an image and between shot cycling) Both have seen improvements with Windows Phone 8.1 (and I'm assuming again when the Cyan update is pushed out) but the 1520 just simply has a faster SOC and it shows, especially when bracketing shots.
The rest of it comes down to size and storage. The screen on the 1520 is simply gorgeous, possibly the best phone screen I've personally ever used (and I'm saying that having a distinct preference for OLED displays). BUT it's a big phone. I don't mind the size as for me it actually serves as my Executive Mobile Device most days and I simply don't find myself breaking out a Tablet/Laptop unless I've got to use a full-blown productivity app. Having the ability to expand the storage to 160GB (32Gb internal plus 128GB MicroSDXC) is a godsend for me and it is one of the few phones on the market of any flavor that (again with WP 8.1) really makes use of that storage should you choose to go there. Once you fill the 1020 up (which my Wife does about every 6-8 weeks) you are stuck moving photos/video/music around.
There is no doubt the 1020 is more pocketable though...
Suffice to say I like both phones, and my wife LOVES her 1020. She takes a dozen or more pics with it a day, does most of her Facebook on it, and more IM that I care to think about. And she has tiny little hands so for her the 1520 would be a two-handed experience. I LOVE my 1520 because it really is the best mobile communications and light productivity device I've owned to date and the Camera is nearly as good (but not quite).
Good luck with your choice,
- Michael