>The first question is do users prefer to pay a one off fee >(even though that fee could be quite large) or do they >prefer to pay a set of smaller monthly fees?

Some prefer one off fee. Some prefer paying in installments. You should offer both if you can.

Monthly payments is better for long term cash flow management of your startup. But it comes with some admin hassle.

Single large payments are better for the short term growth of your startup. And it has no admin hassle.

So why not offer both?

3 month trial is a long time. Only offer such long duration trials if you know your service is very sticky but has a long learning curve. If the learning curve is short, don't offer more than 1 month trial. If your service is not sticky, don't offer a free trial at all. A demo will do.