Portofino vs Portovenere?

Between the two, I would choose Portofino. Next to Santa Margharita and a short ride to the Cinque Terre. Portovenere looks like a town very similar in style and paint colors to the 5 next door in the Cinque Terre. It's pretty but not that large. But, for fun, I would add Lerici for you to consider. Very scenic and beautiful. Quite surprising. It's across the bay from Portovenere and can be reached by bus from La Spezia.

I haven't been to Portovenere, but here's my reaction to Portofino. It's a scenic town without much to do. My old Blue Guide calls it a "haunt of rich yachtsmen," which I think is a fair assessment. I suspect that staying there would be very pricey. If you want to stay in the area, you should consider S. Margherita Ligure, as well.

Both beautiful, but both limited by location as they are at the end of peninsulas and not so big. Portofino is more precious and has fewer side trip options. Portovenere has ferries to Cinque Terre and Lerici. Both have hiking choices. Lerici is a good idea and so is Santa Margarita or Camogli. You'll have a good time with any of these choices.

Having been to both Portofino & Portovenere, I'd opt for Portovenere if I had to choose just between those two...but why not stay in the one of the CT towns instead? Portofino is very small and doesn't offer a lot to do and the transportation options between Portofino and the CT towns are sparse. For example, there are no direct ferries between Portofino and the CT and no direct train either. It's easy to get to Portovenere via ferry and it's slightly larger than Portofino. But, truthfully, I'd rather stay in a CT town and be able to get back and forth between the other CT towns quickly via train. Have you already been to the CT towns and therefore want to stay somewhere different this time? Having been to the CT twice and to Portovenere and Portofino once, I'd still rather return and stay in one of the CT towns vs. staying in Portovenere or Portofino. There's just more to see/do in the 5 CT towns and you can get one from town to the next fairly quickly.

If you're set on wanting to stay somewhere different, you might consider Camogli. Camogli is served by train so you can easily take the train to/from the CT towns (though it takes about 45 min to an hour each way) and then from Camogli, you can take a bus or boat to Portofino for the day. In my opinion, Camogli is just as picturesque as the CT towns, Portofino and Portovenere. Just some food for thought!