Looking for recent opinions on whether one is preferable? The son of a friend is going to spend the next five months near San Francisco for an internship and the pass would be a Christmas present. Thank you.

Curious myself I googled "san francisco go card vs citypass" and found there are a number of sites evaluating that very question. Rather than reinvent the wheel you might have a look at those.

I looked at the websites for both. Prices are fairly similar, Go Card offers more choices but some of them are kind of cheesy, and some, like Madame Tussaud's and Ripley's Believe it or Not are not unique. On the other hand, per their FAQ, the CityPass only gives you 9 days from when you activate the pass by visiting one of the attractions before it expires, which could be a drawback if the son is working.
You should probably look yourself, see if the attractions are things he would be interested in and read the FAQs.

In case anyone is looking for similar information I will just add that I ended up getting a San Francisco Explorer pass, a form of "Go Card," but I ordered it through Costco. The Explorer pass will be good for 30 days from activation, which will give the recipient more flexibility. If he were on a shorter timetable it would really be six of one, half a dozen of the other. The reason I ordered it from Costco is that the Go San Francisco website does not have a "gift" option; if I ordered it directly from that site the pass would have had my name on it. Not a problem for a family traveling together, but potentially a hassle for someone other than the purchaser who is traveling alone. However, ordering it from Costco gave me a "gift" option and allowed me to put the recipient's name on the pass. I think Expedia also had a "gift" option but the card would have been activated instantly, which I didn't want.