I have the impression that there’s a higher rate of “doesn’t work for me, I QUIT” incidents with Minimed. But they work pretty good on a lot of people, and Dexcom has frequent “this sucks, I QUIT” events too.
The only way to know which works better for YOU is to try both-- preferably, at the same time. Dexcom sells with a “money-back” guarantee, but Minimed doesn’t, so you’ll need your Endo to work a loaner/trial for you.
Minimed can use a 522/722 pump as the “monitor/receiver”, that’s very convenient if you already own (or are about to buy) a compatible pump. And the Minimed receivers (stand alone “Guardian” or pump) are quite waterproof if the seals are in good condition, while Dexcom’s receiver is not- splash it on the wrong place, even for a moment, and it’s dead. (Wrong places are the control buttons, and the data port on the side, and the hole to open it for servicing on the end, so there’s “wrong places” all over the durn thing.)
But other than that, and the much higher Sensor prices, Dexcom is nicer: the Transmitter doesn’t rub on your body, the Sensor taping pad holds up a lot better, and for many people (including me) it is way more accurate.
But YMMV! It’s very possible that NEITHER will work for you, so don’t put your money down unless you can get it back!
If you’ve got “good control” with no problems, I wouldn’t bother. Really! I have hypo unawareness and strangely flying bG levels (not related to food intake or insulin, probably something broken in my liver), so I need one. But they’re expensive and aggravating and not anywhere near 100% reliable; if you can get by WITHOUT, wait for much better future models. (NOT Navigator, it’s NOT really any better than the Dex.)