I have an iPhone 4 and an iPad2 that I upgraded at launch from an iPad1. I suppose you're kinda in a tricky spot, being that 4S has just released and next year we'd probably be seeing iPad3 and iPhone5.
I was personally planning on holding off on upgrading to an iPad2 and wait out for the iPad3; however when I made the jump, I never looked back. The upgrade was significant for me. I am able to 'use' the apps without crashing, for one. I also use my iPad2 heavily (when I travel, I just take my iPad2 and a keyboard in lieu of a laptop) so I may be a little more sensitive to these changes.
Both are great buys in my opinion. While I would love to get an iPhone 4S, I'm strapped for cash after purchasing a new MacBook Air, and my iPhone 4 does everything I need/want it to do (except for Siri, of course) right now, so I think I'm holding off until 5.
If I was in your shoes, I may be more inclined to buy an iPad2 than an iPhone4S. I don't know...personally, if I buy my iPhone 4S now, I'd have to sign up for another 2 years with AT&T, and if/when the iPhone 5 comes out next year...well, I would be paying another early upgrade penalty for it. As for the iPad2, however, there will still be a decent market for it even when the iPad3 rolls around (I sold my iPad1 at a good price and used the fund to pay for my iPad2), so my vote goes for the iPad2...by a very small margin.
It's a tough choice for sure

Best of luck!