Does the cofounder have amazing skills or other benefits to the company so that he's worth more than might be expected?
Has Joe been kind of slacking this entire time, not having done much work or taken much risk, while the cofounder expects to put in hundred-hour weeks and is quitting a high-paying job to take the offer?
Interestingly enough, one of the reasons i enjoy startups, is because it enables one to make decisions that are less constrained by big company style black or white parameters. I.e. When share options are discussed in a big company, the deal is often seen as a zero-sum one. If A gets more shares then B gets less. A gets wealthier and B does not.
In an ideal situation (that is far more visible at small companies and startups), giving A more shares could well result in increased value for B too (since the company value without A drops dramatically).
In an attempt to answer the question somewhat better though, i would suggest you ask yourself not "what have i done till now", but instead, "how critical to our future success is the new guy".