I9500: Octa-Core Exynos model. 3G only. Comes with warranty in Pak. Available for 32-33k NEW.
I9505: Quad-Core Snapdragon 600 model. 4G LTE enabled. No official Samsung warranty. Available for around the same price. Black Edition is based on both I9500 and I9505 (depending on which one you choose). Black Edition is slightly more expensive (34k I think).
I9506: Quad-Core Snapdragon 800 model. Not entirely sure if it supports ordinary LTE or not because I9506 is an LTE-A device, LTE-A isn't available in Pak so idk. 3G will work though. I9506 is a great device in terms of performance.
I337: AT&T device. Specs are identical to the I9505. 3G will work, but 4G won't.
I545: Verizon CDMA device. Same as I337. 3G will work, 4G won't. The bloatware on VZW devices is slightly annoying.
M919: T-Mobile Variant. Same as I337. Nothing special. 3G = YES, 4G = NO
Now, let's talk prices.
I9500 is usually around 32-33k as mentioned above.
Same goes for I9505.
However, BE is slightly more expensive.
I9506, is relatively cheap, because demand is low, so you can have one for under 30k. Easily available for 25-26k.
I337, I545 and M919 are all US models so they all have the exact same price. Can be had for 24-25k. Depending on condition.
Hope this has helped.
If you don't care about warranty, get I9505. @muzammil73 has a I9505 Black Edition. It's a beautiful device and a great daily performer.
Sent from my Nexus 5