If the forced sequence seems arbitrary that can be de-motivating. If learners see its value (ie helping them build understanding) it can work.
Without seeing the details of what you do, hard to comment. But often the choice works better the other way round. ie they choose the section to work on (A or B) but within each section a fixed sequence might be used to help develop understanding.
Very dependent on the learners too - if they are autonomous learners I might suggest a sequence but I wouldn't force it. If they need 'hand-holding' a conditional sequence can be helpful. Of course you could give them the choice of which mode to work in.