I did a write-up on here recently comparing the two, but I don't think I mentioned this factor in the comparison.
Heel release is not nearly as bad as you might expect, however, the fact remains that - to get the mag out of the gun you have to use two hands. Only at that point can the weak hand go for the spare mag.
With the P7M8 style ambi release, as soon as the slide locks back on the last shot the weak hand can go for the spare mag, and the strong hand can drop the mag on its own, making the gun ready to accept the loaded mag.
This is all in theory, of course. In reality it likely makes little to know difference, including in timed events... but it theory having to use both hands to get the mag out should be slower.
I always thought heel mag release was a Euro thing: P38, P9S, original SIG 220, PSP, etc. But then I got a Luger... it looks and feels more like a 1911 mag release.
Everything P7 is getting harder and harder to find. The best advice is just keep searching the EE and elsewhere -keep your cash ready - and when the right gun comes up... pounce! Don't settle for what you don't really want by telling yourself you need it now. You'll just end up selling that gun later.