the plus side of a roku is that even a moron can work it. the negative is that it has no browser, can not use a keyboard and requires roku updates to get new content. roku does have the largest selection of channels in any of the streaming players though.
other streaming players such as the one by sony do include a browser and sometimes local channel support which is a plus. the negative is that they have less updates then roku, less channels and are slightly harder to work.
google tv (which is android based i believe) can install apps and use the internet which are plus features. negatives are that it has even less channels then the prior two and is rather hard to work. i have a custom rom on my phone, build my own pcs and in general am very tech savvy and i found the whole one piece remote-keyboard-pointer very annoying and irritating. kudos for the idea but not very practical at this point.
another option is of course to use a pc. this gives you the ability to go anwhere and view any content as well as having a much more powerful platform. your parents could then write documents, check email, surf the web and watch tv on the same platform. this is the most powerful option but not the most user friendly. with so many interfaces and settings i would suggest this only to those who are computer literate.
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i would say roku2 xs for your parents. if they want web then the sony streaming player (not sony google tv version).
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generally speaking youtube is NOT that important especially for older people. netflix, hulu and possibly amazon are better choices.