There is a bit of an argument about this whole process, with some claiming that Google Now is better than Siri because it was built "from the ground up" by Google (which isn't wholly true), whereas Apple just bought Siri (which also isn't quite correct). It's all a silly argument based on geek pride, but it did get us looking into the process by which these products came to be what they are.
We can't really get behind the argument that one way is better than the other, because it quickly becomes a slippery slope. The problem is that at some point, an acquired product can't be attributed to the original creators anymore, even when the original creator comes along in the acquisition. The prime example for us is Android itself. Android was created by the Android team headed by Andy Rubin, then acquired as a fully functioning product by Google. Andy and the team still work for Google, but Android has become a wholly Google product (ignoring forks and NGAs, of course). Similarly, Siri was bought by Apple as a fully functioning product, but to say that was the same app that now anthropomorphizes the iPhone just isn't correct.
The bit about Siri
The point is that for better or worse, Siri is nowhere near the same app that it was when Apple purchased it. Sure, the backbone of voice recognition powered by Nuance is still at the center of Siri, but what the app can do has changed dramatically. What we see in iOS is no longer the product of the Siri developers simply purchased by Apple and stuck into iOS, this is an Apple product with features and uses dictated (no pun intended) by Apple.
Now... Google Now
On the other hand, Google didn't buy one singular product and transform it for Google Now. As we said, this product/platform is more of a Frankenstein monster made up of a number of acquisitions, hires, and in-house development that can be traced back at least 5 years. In many ways, Google Now is an initiative similar to Google+, which is intended to be a platform to unify a number of existing Google products and services, including search, places, travel, suggestions, and of course speech recognition.
On the other side of the voice coin, Google also needed a nice voice to respond to users just like Siri does. Luckily, Google had purchased Phonetic Arts back in December of 2010. That purchase was originally made to make robo-voices sound better in Google Translate, as well as the accessibility text-to-speech option found in Android. Phonetic Arts did a lot of work making robo-voices sound better, and that technology has come in very handy with Google Now, because it does sound like the assistant voice is smoother than Siri. As the voice database was growing to a sufficient level, Google also needed to beef up search results.
Flight information is gathered via searchable results from flightstats.com, but eventually, Google could move to its own information repo which it purchased in the form of ITA Software (acquired July 2010). ITA Software now powers Google's Flight Search for tickets, and that could easily become part of Google Now's results. Place results, which are a huge part of any mobile search product (because more and more "mobile" really just means "local") have also gotten a boost from a couple choice acquisitions. Google hired the entire team behind Ruba.com (May 2010), an online travel guide, in an effort to give local results a boost. Then, more recently, moves to purchase Clever Sense (December 2011), makers of local recommendation app Alfred, and restaurant guide Zagat (September 2011). Google notably had a falling out with Yelp, and the purchase of Zagat was the answer to that loss of local data.
Metaweb, which Google purchased in mid-2010. Apple of course has to partner with information services like Wikipedia, Wolfram Alpha, and Yelp, but it's Google's business to provide organized data and knowledge to users, and Metaweb is the future of that endeavor. Metaweb just recently made its debut within Google in the form of Google's Knowledge Graph and Semantic Search. Now, rather than searching for keywords, we're searching semantic objects, which means there should be better differentiation between homographs (so Google now knows whether you're searching the meteorological "thunder" or the NBA Thunder.) These results have shown up in the Knowledge Graph block to the right of standard results.
These blocks of information have all made the pretty easy transitions into being "cards" in the Google Now UI. And, on that topic, we have to mention the hire of Matias Duarte in May 2010, who has been one of the strongest forces behind getting the traditionally engineer-oriented Google to make well-designed products. Matias has always loved the "card" metaphor, and so we feel pretty safe in assuming the cards in Google Now were his idea. For those of you that ever used webOS, you know that cards were a central metaphor of that platform (designed by Matias), then we saw the multitasking menu of Android change to cards when Matias arrived and now we get the cards of information as part of Google Now.