Like others have said, you're essentially getting the top of the line BB from each of the carriers, so the differences will be marginal.
Here's a comparison of the two on BB's site, and if you look thru all of the categories, you'll find besides cpu ( 9700 is faster ), they're virtually identical, EXCEPT it appears the S2 can handle a broader variety of bluetooth functionality, while the 9700 has a few more codecs available for playing different types of media formats.
BlackBerry - BlackBerry | Wireless Handheld Devices, Software & Services from Research In Motion (RIM)
I've used a Bold 9000 for the past year, and hands down, it's the best smart phone I've ever used. And I've used a lot. It doesn't have the best browser, but with current beta's it's quite good for what I need it for. The one thing is, the phone ALWAYS works, and works well. Speakerphone, earpiece, dialing, conference calls, messaging, etc. For work, it's as reliable of a remote device as I've come across. And I use it for a lot of email and messaging. From the reviews people have posted about the 9700, it even edges out the 9000, with the big differences being a higher resolution screen, albeit smaller phone form factor ( so some people with big hands don't like it's smaller size ). My problem is, I'm on AT&T and their network in my area has been absolutely unacceptable lately. So I'm going to switch.
That leaves the S2 as my real option on Verizon, the Tour on Sprint, or the 9700 on T-Mobile.
One thing to remember is, on a CDMA network ( verizon / sprint ), you won't get simultaneous voice and data in the same way you would on an HSDPA enabled 3G GSM network ( AT&T or T-Mobile ), unless you turn on wifi, and log into a hotspot before making any phone calls. If that's not important to you ( being on a conference call while trying to send either users on the call an email, or other people, or use MSN IM / GTalk / Blackberry Messenger / etc WHILE on the call ). If you're a user that sits on long conference calls regularly, it's something to consider.
Finally, for me, the biggest concern switching to an S2 is lack of physical keyboard, and available shortcuts. But, most people say they can type nearly as fast on an S2 with practice as they could with their Bold / 8900. And, the lack of keyboard shortcuts is made up for, because navigating the OS is even faster than it is on the Bold / Tour / 8900, rendering the keyboard shortcuts useless ( which are available on the S2 via pulling up the keyboard and hitting those keys, OR via some app like QuickLaunch ).
As far as screen goes, my eyes do get tired looking at my 9000's 480 x 320 screen, even tho the quality is excellent, so the S2 is somewhat appealing, even tho resolution-wise, the 9700 and S2 are identical. I don't think I'd want my next phone to have as small of a screen.
Best of luck, and report back with what you decide and report back your experiences.