Stock wouldn't make a lot of sense for Basecamp though, unless they started paying dividends or something. They aren't public and have no indication that they ever have any desire to be public, and the ownership of the company is small enough that I'd be very surprised if any ownership stake didn't come with a lot of conditions on how that ownership could be transfered. As a liquid asset, Basecamp stock probably isn't particularly valuable.
They said they'd split part of the sale proceeds, should it ever happen, with all the employees. There's no way the expected value from that would measure up to years of Amazon RSUs. But then, that's the case for almost every other company attempting to pay top of market.
"There are no plans to sell Basecamp or take the company public! But, in the slight chance of that happening, employees may be eligible to receive a portion of 10% of the value of the company if Basecamp is sold or made public. That 10% would be divided into shares, and shares distributed amongst employees based on their tenure at the time of the sale/offering."