MichaelBarg is correct. Windows Phone isn't comparable to past versions of Android where throwing faster hardware into a phone will give you better OS performance - Windows Phone is pretty much smooth out of the gate on low to high end hardware. There isn't much differentiation in hardware because of how Microsoft dictates and limits spec ranges to manufacturers. Microsoft is closer to Apple than Google in this regard, with the advantages and drawbacks of a guarded ecosystem.
A quad core CPU in a WP8 phone like the Lumia 928 might benefit some demanding apps, maybe, but there isn't a lot of proper multitasking or many very demanding apps that aren't more graphical in nature, aside from some benchmarks.
Blue/8.1 is for desktop. I haven't heard anything about what the WP equivalent is yet. The "blue" designation is supposed to represent the new changes coming to desktop (Windows 8), tablet (RT), and phone (WP8), but the phone OS changes are behind the desktop changes as far as I can tell.
I wouldn't hesitate getting a Lumia 928 if you know you want a phone with a good camera. It's essentially a Lumia 920 but with a Xenon flash. Windows Phone 7/7.5 has held up fairly well, I still use my old Trophy occasionally even though it's 2 years old. Doesn't feel sluggish, and I have a HTC One to compare it to. Windows Phone scales better with lower resolution screens, it gets really easy and comfortable to use the live tile system with hubs.
Just know the advantages and drawbacks of Windows Phone especially if you don't like Apple and are an Android fan. Windows Phone isn't like the old Windows Mobile, which really was as customizable as Android now is (custom ROMs and tweaking was old hat for WM long before Android came out). The word "Windows" in "Windows Phone" is very misleading because there aren't actually any windows in Windows Phone, lol. They should have named it Tiles Phone.

It's great, though, if you want a phone that... uhh... "just works".