I only really recommend the EZ4 mostly because it's what I have. While I've heard the M3 is fantastic, it's basically extinct. Though really, both are great in their own ways, honestly. EZ4 definitely has the more usable UI with little bloat, but the M3 has some extra features built in, which aren't particularly useful now but are still there. It all comes down to availability though; EZ-Flash 4 is available pretty much anywhere for a great price, but the M3 is, as said, non-existent essentially. But if someone can find one of those cards, by all means try and grab it, but with the EZ4 out, it's not worth the hassle trying to find an M3.
Just my opinion though.
(Also, the EZ4 DOES still use a battery to store the temporary saves before transferring to the cart itself, but it's not as big an issue as a consistently used RTC, at least I think)
I agree. Basically extinct. No one who owns one would relinquish it, and if anyone did, it would be virtually impossible to run into a person selling it on the internet.
EZ4 is definitely the way to go right now.