So you're leaving an OS because of a manufacturer (but not the one that makes the OS)?
Sony phones will come with an unlocked bootloader
Sony Ericsson to Unlock Bootloader in Xperia Models | PCWorld
This probably includes the Xperia Play which should be on VZW soon.
Motorola is following suit:
Motorola Will Make Its Phones Easier To Mod | ITProPortal.com
Motorola News: Atrix May Get Bootloader Unlocker Like Motorola Xoom's - WIRELESS AND MOBILE NEWS
Funny you say you won't have access to 4G so you don't get a 4G phone, but you are "reluctant to get a single-core device." You do realize this is like buying 8GB of RAM to use on a 32 bit version of Windows, right? You need the program to utilize both cores. The benchmark test used to not even incorporate the second core of the XOOM in the test results. If that app doesn't use it, what makes you think you'll see the benefits of dual cores in other apps which, most likely, won't optimize dual cores?
Also, 4G is not extra. Nor is it capped on VZW. Yet.
This isn't to keep you on VZW or Android. Quite frankly, I may get the iPhone 5 on AT&T this Fall, if it truly is a global phone as it is rumored to be. It will be dual-core and knowing Apple, they WILL make sure that native apps, as well as other big-name third-party apps will utilize both cores. Much like they had apps optimized specifically for the dual-core iPad 2. FYI, the iPad smokes the XOOM in just about everything (insert Flash joke here). That's why I too made the switch from Windows to OSX. My switch to the iPhone? It's more about me leaving the States for two years and living in the Caribbean and the carriers there using only AT&T band frequency phones, but still not AT&T themselves. If my upgrade is early enough, and the iPhone 5 comes to VZW (I can't imagine it not), it might be a no-brainer. I don't want the Droid 2 Global then. Though there is a rumor the DInc2 could be global.
And I could go on and on. You have an Eris. I did too. It sucks, I know. But it's your hardware that's holding you back. Not Android. And as hard as it is to believe, not the manufacturers either. You haven't even experienced the benefits that lie between the Eris and a locked bootloader yet. And the later is going to change soon, anyway.
If I leave Android, it's because I'm fed up with unfinished updates to the OS and a lack of hardware optimization. But it still beats iOS with it's functionality and, as weird as it may sound, widgets.
About your fear of carriers stifling root users, it's nothing at all. They can not legally do anything to ruin your service as per your contract. If you illegally wirelessly tether, well...you are breaching the contract. Rooting your Android phone or jailbreaking your iOS device is not against the law.