So I've played a fair bit with Siri on an iPhone 4S and Google Voice on a Samsung Galaxy S2. I like to think I'm reasonably objective - I have several friends who work for Apple and friends at Google. I think people being over-the-top fans for technology companies that they don't actually work for is actually a bit crazy...
That said, Siri has the advantage. In terms of searching they are similar. I thought they both did a great job and I didn't see many mistakes in either one. The results from both were good. I sent one text message with Google Voice and I composed an email with Siri and that went fine on both. They both didn't make a single mistake - although the text was very short - and I was impressed with both.
So for these, I thought they were equivalent, but Siri can do a couple of other things.
Siri can do location based alerts - and this could be really useful while driving. I particularly think the "location-based alerts like "Alert me when I get home to take out the garbage" and then when it sees you are home, you get an alert. It's really neat (and a bit creepy) and also really useful. And the whole calendar thing is equally useful and Google Voice can't do that either. And Siri will often read you the answer, while Google Voice navigates you to a webpage. Since I'm all about using this in a car, speaking the answer back is thus safer (and cooler) than taking me to a webpage.
The big use that I see for both is driving. I have bluetooth in my car and I have a button to activate it. I can hit the button and talk to my phone and have it do things and thus I don't need to mess with my phone while I'm trying to drive. I have literally almost killed myself texting while driving - in fact one time I almost drove off the onramp of a bridge and it was a very close thing. I personally spend a fair amount driving and I personally have a horribly bad, really stupid habit of messing with my phone while driving. I also have a mandatory screen unlock on my phone (company's security policy) and so even simple things require a whole bunch of steps. I also ride my bicycle a lot - during warmer months I usually ride over 80 miles per week. So, again, getting out a phone and punching in a big long passcode and then typing something while riding a bicycle is both hard and somewhat suicidal.
People can say Siri is silly, and that talking to your phone is stupid, and that people have been drinking way too much Apple kool-aide to be excited about Siri but from my perspective both Siri and Google Voice have the potential to make my life safer (no more texting while driving) and easier, and I think everyone needs to judge for themselves how useful these things are. I think anything that reduces texting while driving is a Good Thing (tm) - whether this is Google Voice or Siri. But of the two, I think Siri is the better of the two only because you can do a few more things that I actually want to do with it and it will reply in voice as well.... although I'm certain that in less than 9 months, Google Voice will be able to do these things too.