My feeling is that 0.5 is what’s most robust and future-proof. In fact, I’d suggest using the nifty rescaling feature to convert your existing goal to hours and then enter 0:50 when you do a 50-minute session and 0:25 when you do a 25 minute session.
The thing you’d enter for the rescaling (at the bottom of the Data tab for those who don’t know) would be “50/60”.
I can definitely see the argument for minding just number of sessions though.