The heel releases are usually much more secure than the button type. I would think if your gun was dropping the mag on its own, there was something wrong with it. I've had a lot more problems with a button type release than a heel release when it comes to unintentional mag drops. If anything, the heel type will usually retain the mag in the gun, even if it were to be released.
With a little practice, the heel releases are not all that bad, and in reality, on either of these two pistols, a non issue. I seriously doubt you'll be trying to do a games type speed reload with one.
Not as much an issue these days, with the number of small, DA/DAO type pocket pistols floating around, compared to what was available when the Walther first showed up, but the DA type trigger makes these pistols viable for safe carry with a loaded chamber. I guess you could argue the Walther is even better in that respect, as its trigger is heavier. If you have a good DA trigger, and have spent a little time practicing shooting that way, its very easy to shoot well with. If you go the Walther route, your probably going to want to sent it off for some work.