I upgraded from a 925. Main reason, I wanted a better camera. The 1520.3 more than fits that bill.
If you're getting off Verizon it will run LTE on AT&T or T-Mo towers so your carrier choice is wide open. NOTHING out there beats 1520 right now for Windows Phone. Nothing is poised for the future better. No waiting for carrier approval at update time. You would instantly move to 8.1 + Cyan (official, no P4D needed) and most likely 8.1.1 + Denim by the end of the year. The 20MP shooter is great - heap on Qi charging without a jacket, 32GB internal storage, up to 128GB microSD, Snapdragon 800 chipset, 1080p IPS screen + ClearBlack display, Dolby Sound enhancement, Quad microphone best-in-class audio for video shoots, Glance screen enabled and that lasts-all-day-long-no-question-about-it 3400 mAH battery...
I looked at it and thought I must be crazy to NOT get a 1520.3. The way I saw it, it was the only worthy upgrade from one proven workhorse (my 925) to another.
You decide what's best for you, but don't let the size of the thing scare you off. I went for it despite the size and could not be happier.