[Anthony G Williams]

The disadvantage of putting a substantial flashback within the main story is that it disrupts the flow of the tale. I like a continuous narrative which grips the readers and takes them along, unwilling to put the book down.

If you have a lot of background to fill in, it can be done by alternating chapters to allow two parallel stories to develop (now and then), but I'm a bit dubious about that approach.

So yes, I think I'd prefer a prologue in that situation, although it's difficult to judge without reading it: the principle matters less than the execution of it.