I think Siri's great. I use it daily when I'm driving. "Read me my texts" and her voice comes on over the Bluetooth system in my car. It's a lot safer than texting while driving.
As for the 4 to 4s debate, it depends on what you're going to use it for. The 4S is a minor upgrade of the 4; when the 5 launches, it's rumored to be a new design style (major upgrade) which typically launches with a few glitches/bugs/etc, such as the death grip bug of the original iPhone 4.
But, on the other hand, you're looking at some major changes like LTE support in the new iPhone. Since you already have a 4, I'd say wait for the (hopefully) LTE enabled iPhone, since you're going to want that speed to take you the two years till your next upgrade.
I personally just got my first iPhone - a 4S - because I was on a piece of garbage Blackberry Curve. And, the way that my carrier works their contract, I can get an upgrade in about 21 months instead of 24 like most carriers (Sprint lets you upgrade on the 1st of the 22nd month; in my case, although I upgraded in late January, I'm eligible for an upgrade on November 1, 2013.)
Hopefully by the end of 2013, Sprint will 1) still be in business and 2) have their LTE rollout completed!
Don't forget, some carriers like Sprint will let you do a "buy-up" to make yourself eligible for the 2-year upgrade price. I think the rep I spoke with said it was $45 per line to reset eligibility. That's not bad if you're happy with your service anyway and it's a lot cheaper than an early term fee!