You won't be able to use Global GSM model on Verizon's LTE since it only support Global bands. On the contrary, if you go for CDMA model, it'll support ALL of those Global bands in addition to Verizon and Sprint LTE networks in the US.
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I assume West LA you mean Los Angeles? If that's the case, AT&T only has 10Mhz of their Band 17 LTE spectrum which will have speeds up to 37mbps in that area. That means once your neoghbours start buying them iPhone 5's and start populating that AT&T limited spectrum, you're about to hate life lol

On the other hand, Verizon has deployed 20Mhz of LTE in LA with speeds up to 73mbps which is gonna take that hit much nicer. Not sure about your area, but here in NYC I can hardly ever complete a call on AT&T, and use my Verizon line whenever I'm making any important call.
It's too early to answer your last question though. Verizon iPhone 5 will cover all the GSM/HSPA frequencies for AT&T's network, as well as GSM/EDGE on T-Mobile and in some areas their HSPA on 1900Mhz they're in process of refarming. If we look at the iPhone 4S, then Verizon will most likely lock down domestic GSM providers from their phone, but if we're looking at the iPad 3, either Verizon or AT&T model played nice with all of our US carriers. Verizon iPad actually accepted AT&T's SIM for HSPA+ access!
We'll see.