Under the hood, the devices are mostly the same.
Differences are minor.
The 930 is smaller and I'd choose it for that reason alone.
However the 1520 has slightly more on its side with a microSD slot and support for two small but popular Nokia features: Glance and double tap to wake.
The 1520 is primarily plastic, the 930 is more of a hybrid IIRC.
Display tech is different, (930 = OLED, 1520 = IPS LCD) but is subjective which is "better"
Also depends on which variant you get. There is a flavor of the unlocked 1520 which is fully compatible with T-Mo US bands which is a big deal for many.