|2. Any gigabit router (including the airport extreme) and a NAS...Option 2 though seems more complicated. I've never set up a network before and also slightly more expensive...is there a third option of hooking up a NAS into an airport extreme
As you note, an Extreme is a gigabit router, and yes, plenty of AVS folks have a NAS device wired over gigabit into an Extreme.
|1. An airport extreme with an external USB drive...It seems like option 1 is much simpler to set up but doesn't have as many bells and whistles (ie. access to storage from any internet connection...
You can access your Airport Disk from the internet, at least Apple say so. I've never done it, but there's a setting tab for it in Airport Utility.
|...airport express only has 1 usb port so I can't keep mulitple drives going
You can keep multiple drives connected to an Extreme even though it only has one USB port, just use a hub, or an enclosure that has multiple drives but connects over a single USB cable. As I mentioned previously, I have 7TB of USB storage on my Extreme.
You can't connect any USB drives to an Express.
What you cannot expect to do with a pool of AirPort Disk storage is serve files up to multiple users simultaneously.
|What do you suggest and why?
Since I'm the only one who has been giving you advice, and based on the questions you've asked, here's what I really think you should do: instead of buying a NAS, buy another Mac instead.
Dedicate that Mac as your iTunes Home sharing and media server, stick big drives inside it or connect drives to it over firewire or USB, wire it to your network over gigabit, connect your aTV2 over gigabit, then sit back and enjoy controlling everything with your MBP, iPad and iPhones.
Keep it on 24/7 if it is energy-efficient like a Mini, or allow the aTV2 to wake it on demand if it is a power-hungry beast, like an old PowerMac. Dedicating a Mac as a whole house media server shares a few advantages of a NAS in the sense that you can put it anywhere, even out of sight if it is too big and noisy, and can easily be controlled remotely. It's easier to set this up than a NAS or an AirPort Disk, too--nothing tricky about sharing media from a Mac to an aTV2, iOS devices or other Macs on your home network--whereas it can be moderately tricky and time consuming moving a big iTunes library over to a NAS or AirPort disk and then managing it remotely as you go forward.
So, that's a roundabout way of getting back to this, if you want an aTV2, my advice is don't be tempted by moving your iTunes media library to a NAS or to an AirPort disk: while both approaches can be done, it isn't as easy or seamless to maintain a single "centralized" networked iTunes library as it should be this way. The web has been littered for years with complaints of loyal Apple users wishing this WAS something Apple would tackle and hit a home run with, but it hasn't been in the cards.
So instead, if you want an Apple TV, reduce complication, don't fight what Apple wants, and what Apple really wants you to do is set up iTunes Home Sharing on a Mac, and then keep iTunes running on that Mac with all your media stored locally. Your overall user experience will be much better this way.