Highrise or Salesforce for early stage startups?

You will also need a bunch of other things. To really get the benefit of automation in SF, you'll need the enterprise version. Then there are Gmail plugins (we use cirrus insight), billing platform integration, etc. etc.

Getting SF configured so it makes sense for you will take time and $. There are also a number of little gotcha's, like inability to gracefully deal with email aliases. Might not be a problem for you, but we find that a number of people use vanity email addresses, personal Gmail, etc. to communicate - and SF becomes exquisitely painful suddenly.

In short, unless SF solves a serious sales process automation problem for you, don't go there.

Ah - also, analytics. You'll want the Good Data analytics on top, especially if you're in SAAS space. Good stuff and greatly increases the value of SF.

I'm assuming you want to manage a high touch B2B sales process (otherwise why would salesforce be in the running?). We're using salesforce for historical reasons. It's been fine for us, but I would caution someone starting fresh: 1) it's expensive, and 2) it really expects to be used by hand by a sales person (and it's good at that) - doing mass changes and other activities that you might expect to be easy as a database programmer are always painful. I haven't used Highrise so I can't compare them.