'What's the difference between an aorist infinitive and a present one?'

Essentially the difference between the two is in what is known as the 'aspect' of the action they refer to, which can be either continuous or instantaneous. For example, whereas 'δει υμιν λεγειν' means 'you must talk' means generally that the people addressed ought to talk to one another in the present or future, 'δει υμιν λεξαι' means that the people addressed must talk at that particular moment in time.