Coinbase and Bitfinex and Poloniex, oh my!

I have created accounts now with Coinbase, Bitfinex and Poloniex.  

First came Coinbase, it was simple to sign up and make a deposit of USD and excitedly purchase my first $100 of BTC. But, when the Bitcoin fork was coming a few weeks back they at first said they weren't supporting it. So naturally wanting to get my share of Bitcoin Cash, I moved my measly fraction of a coin over to Bitfinex. At the time, I figured that would be my new primary exchange, but it appears they don't accept deposits in USD, which is what I would be using to buy Bitcoin and other crypto currencies. So I don't think it will be very useful for me. 

A week ago I became interested in NEM, and realized neither Coinbase nor Bitfinex trades it yet.  So today I set up a Poloniex account, but in the process they asked for my social security number and an identifying photograph! I logged out at that point, feeling creeped out and needing to think about it further.

Now, one would think I could just go back to Coinbase, but in looking at the numbers, at this moment Coinbase is wanting $4,664 for BTC while Bitfinex $4,542 and Poloniex is $4,500. For a small timer like myself who is only looking to invest a couple hundred bucks every now and then when I can, that much of a margin feels significant. 

So my questions are, what's up with Poloniex wanting that info from me and is it a good exchange? Why does Coinbase want so much for BTC? And does Bitfinex really not accept USD, or am I just missing something? Really I have a lot more questions, but we'll start with those.  

Obviously I'm new to this. I realize by not knowing what I'm doing I could be throwing money down the drain, so any advice is appreciated.  

Thanks,
Buzz