Should I ask for deferral before accepting a PhD offer or after accepting it?
Congratulations on your offers.  This is really four questions.


Would I benefit from a deferment?


It sounds like you feel bad about "abandoning" your current program.  You should not.  You probably owe them nothing.  PhD programs benefit universities more than students.  Completing your master's will do nothing to change the fact that you are switching programs.  There is no reason to defer.


Should I ask for a deferment so I can finish my masters?


No, this is not a credible reason to defer unless the masters is required for admission to the new program.


Should I ask the new departments I have offer from if they allow me
to go back and finish my current program and defer my enrollment in
their program until next year, or should I accpet the offer, and then
ask them?


An important rule of negotiation is that you negotiate before you agree.  Accepting and then negotiating is dishonest, and more likely to fail.


Does it make sense to choose based on whether they allow me to do that or not? (and so, ask now)


No, enroll in the best PhD program you can.