My younger brother has the a71, and he complains its laggy as hell on gaming. Esp on ML haha. But other than that its a great phone. Really large beautiful display. Okay camera.
I am a happy owner of the A71. Both are identical except the M51 is the later release and a huge battery with the added weight. If standby time is your thing the M51 looks compelling.
For long term Samsung users, are your phones doing ok even if you have no more updates? Or is it slowing down? Contract is up and choosing between Samsung and an iPhone.
How long do you plan to keep it? Longest we've (myself and my wife) used a Samsung phone (S/Note) is 2 years max and it was ok. *** With old Android devices, I noticed that the phone (and basic functions are ok -> Call, SMS, FB and IM's) However, you feel the age of the device when you start using the other apps (Waze and G Assistant included).
If 3-4 years? Also re battery - current flagships of both brands seem to have same battery life even though lphones have smaller ones (based on YouTube reviews). Is this accurate? And is handling of multitasking still better on Samsung (keeping apps open in background)?