Were I in your position I would stick with the samsung fascinate.
I personally (OPINION ALERT RING RING) do not like itunes.
I hate having to manage my data through itunes.
And that was on an ipod I had many years ago. I can't imagine trying to manage phone contacts and everything else via itunes.
Aside from that, the hummingbird processor pwns the A4. The iphone is scaled back to 800 mhz while the fascinate runs 1 ghz.
If you enjoy the widget options on your fascinate you can kiss them good bye as iphone 4 does not have any of them.
Plus at the moment IP4 does not have and may not get swype for a while..
Now with that said..The fascinate requires micro managing things to keep battery life up while the iphone 4 takes better care of itself.
Keep in mind that for every pro of the fascinate there is a pro of the IP4 and for every con of IP4 there is some con to the fascinate. So at the end of the day it boils down to waht you prefer.
If you want a experience that feels more like a desktop computer, go with fascinate. If you want a smartphone that has the power to check email, watch a few videos and you already use itunes you will most likely prefer the IP4, because it still feels like a "phone".
Simplicity has its advantages but, I personally want more power and more options in my phone if I were paying 200$ + for it along with data.
As far as networks go that depends entirely on your area as to what coverage you are going to get.
It is my understanding that at&t is rated with worst customer service.
I would stick with verizon in your shoes (assuming you have good coverage) however, I want a more desktop like experience personally. Plus verizon still has unlimited data... if I have to have a data plan.. I would want 'unlimited' and I would make sure I used 30$ worth of data every month.