Which should I pick: 5% equity or $100,000 performance bonus in an early stage startup less than two years old?

I have the option to take either 5% equity (vested over 4 years) or a $100K cash bonus (paid over 2 years) in a startup that I joined a year ago. The company is less than two years old and we have a decent product (it's nothing revolutionary, but the company has some traction).

Which option would you personally pick and why?

The fact you're asking is actually quite telling. I suspect that deep down you know what you're going to do and are seeking some validation. You should do whatever feels right for you, in your situation.

Some things that you might like to consider:
How much would 100k change your life over two years?
How committed are you to the company/product/clients?
Do you believe that you would stay there for 4 years even if there wasn't the vesting handcuffs?
What do you think the company will be worth in 4 years?
Do you think that you would cash out after vesting, or stay in for a longer haul?
What skillset are you, and do you think there's enough room for personal growth over the period?
Are you likely to be poached, or persuaded to join another company in that time?