Interface Language: Simplified or Traditional, that's your choice
You can select add simplified Chinese to your languages and also add it in your profile. Then the Google interface will be in simplified. So Google will interact with you in simplified. And the reverse is true for traditional. That covers how the Google interface interacts with you.
Queries returned: it depends on your input script and is always limited to Google reach
Now about the queries being returned. If you type a query in simplified, the results will be mostly from simplified Chinese sources. Bear in mind that simplified sources are in China proper. If you type a query in traditional script, the reverse is true, the results would then mostly be from Hong Kong and Taiwan. So it depends what you want. Should you want China proper results, use simplified script in the query. If you want Hong Kong or Taiwan results, use traditional script in the query.
Google reach in China is very limited: want real Chinese sources? Use baidu instead...
The truth of the matter, however, is that Google is not good for returning sites from China. And the Google maps for China are not accurate. The street level views also don't work. If you want to know about China, use baidu.com The sites this search engine returns are good and the maps are accurate and street level view works like a charm. You can use Baidu from the US without any problems. Some non-Chinese speakers while in China insist on trying to use Google over vpn (because Google is blocked in China -that is another story-). I personally don't think it's worth the trouble; the samples Google returns about China are not anywhere as useful as Baidu.
Google could be a whole lot better for Chinese
I think if Google wanted to truly make a useful product, it would provide an option to switch the delivery script on the fly, no matter where the source is. In essence, truly treat the simplified/traditional as a user-selected delivery font. So if the source was say, Hong Kong (in traditional) but you were a simplified script user based in the US, you could still read it with ease. And of course the reverse could also be done. That would open up the mainland to 华人 (Chinese established outside China for generations, therefore still using traditional). Or maybe you could be a Google user in China using Google over VPN to read Hong Kong sites ;-).
Anyway, the bottom line is that Google's hits from China sources are not anywhere as good as Baidu's. So, it's not just about script...
Traditional vs Simplified is a complex question
I speak Mandarin and have traveled a lot to China and also lived in China. I learned Chinese in the US using simplified script. Most schools in the US rightly teach simplified script because that is what is used in China. Bear in mind 1.3 billion people use simplified script in China. So it is important to understand that the vast majority of Chinese speakers in the world today use simplified script. Also, the truth is that many outside China people of Chinese
origin (华人, huaren) actually cannot hand-write Chinese anymore. This is certainly true of American Born Chinese (ABCs). Also, another aspect of this, is while traditional characters are "in use" in Hong Kong and Taiwan, one has to realize that in these places, unlike China, English is essentially on par with Chinese. In other words not only do most people also speak English in Hong Kong and Taiwan, they use English in their daily lives. English is everywhere on public signs. English is prevalent at the working place. For example, English is the language of work in Hong Kong. You can live and work there and not speak one word of Chinese. This is not true of China. The fact that in Hong Kong and Taiwan, English is the de-facto language of education/work/business etc. has an impact on the effective literacy, to be more precise, on the ability to hand- write Chinese characters. In China, the ability to hand-write Chinese characters and to use Chinese is complete, and is never competing with English on an on-par basis. This high level of education relies on the simplification of the script. This is why simplified is such a good thing. It has been extremely successful. This is perhaps one of the most successful social advances that China implemented. Bear in mind that the Chinese written language has undergone many modifications in its history; ancient Chinese characters are very different than so-called traditional ones. So the issue of traditional vs. simplified is rather complex.
Limiting your view to one script is limiting your reach into Chinese diversity
I read some comments here that basically go along the lines of "I don't want to see simplified when I do my searches etc... etc..." Again, this is not simply a font issue: if you limit your view to traditional script, essentially all you see is Hong Kong and Taiwan material. So you don't see anything mainland Chinese: you don't really see China. The issue is whether Google, despite having itself kicked out of China, can find a way to get back in somehow. Facilitating segregation of material is not the right way to go I think. Actually, Bing.com is doing a pretty good job in China, they don't seem to have any problems there -they found a way to work while respecting Chinese law-, so maybe Google could learn a thing or two from the competition...