Exclusively Paypal. I've had nothing but extremely positive experiences, but form what I hear, when it goes bad, it goes really really bad. I'm looking into other options
1) When you collect money long before a user might file a chargeback - like if you're selling tickets for an event that will happen in 5 months, or if you're selling a product that you'll be shipping much later.
2) When your 'pattern' changes dramatically. eg, you've made 5k/month worth of sales for the past 2 years and this month you've suddenly sold 20k.