I am using a 1020 now, and if you were to ask me, I would say both are good.
The 1020 camera is powerful. I mean, when used properly, damn, the pictures that come out from it is amazing. Screen size is okay... But compared to the norm nowadays (5" and above), it does seems kind of small. The lack of microSD is not a big deal for me, but maybe if you have tons of music, space may be an issue. Otherwise, the 1020 still can stand amongst the phones out there nowadays. One drawback I would say would be the hardware is starting to show its age. It is still fast, but, newer features that Nokia introduce are not supported by the 1020 (i.e. the Lumia Denim software update. The feature to record 4k like videos and snap the stills will not be available on the 1020). But otherwise, it is still quite a good phone.
The 1520, on the other hand, is the more modern Lumia out there, and ( still) one of the more powerful Lumia out there. The screen is gorgeous, the hardware is fantastic. Although the size ( 6" ) may be a little big, but I think you can get use to it after some time using it. The modern hardware inside it (Snapdragon 800) will ensure longevity on the phone, and newer features will be able to work on your 1520. Battery life I heard is fantastic, so yeah. Camera will not be as powerful as the 1020, but well, they both balance each other out !