30-Day Free Trial, means you collect money after 30 days. This is great if you have a very sticky product or your customer acquisition cost is low. You will for sure get more sign ups because there is very limited barrier to entry. However, the downside is most of the users will not turn into paying customers. So you have to be make sure your product conveys value quickly.
30-Day Money Back Guarantee, means you collect money now! This allows you to collect the cash up front but have to work hard to keep the customer happy (so they don’t cancel next month). You will get less initial sign ups because it requires the customer to take out their billing information. This method is good if you have a sales team and your product is at least $100/mo. This method will require some convincing which can be done in the form of a sales demo.
Both methods look similar but are actually 2 very different strategies. I first learned about the Money Back Guarantee Strategy from Jason Cohen, here is the interview where he talks about it.