Both are equally effective, both have similar side effect profiles.
But they stay in your system much longer than the stated period (allows for maintaining the minimum therapeutic level on the 28th / 84th day.
So if your father really had a problem with the medication, better to start on the 28-day, to limit the problem!
After that, for most people (if not all, but I haven't dug that deep!), the 12-week version is just as effective, but more economical for the NHS, and less limiting for your father.