I'm wrestling with this issue myself on a (broken tap) project. I'm no expert but seems like there several factors in the mix. Even by saying 'spiral flute', there are different degrees of spiral helix. This link suggests angles as a function of material.
And there are different flute counts among similar spiral flute varietals. I've also heard that for tapping aluminum, uncoated/bright is better than coating & is more important than angle. I've heard that 2-turn-in + 1-out routine is not what you are supposed to do with topside ejecting taps, that actually pinches off the swarf & causes chip jamming.
My machinist buddy deals with deep hole copper which is another bugger material & his tap salesman said he was tapping too slow which was counter-intuitive to him. He sped it up & breakages decreased. Who knows, I have great runs & then the tapping gods decide to wreak havoc (proportionately related to how much work has gone into the part!).
Bottom tapping to me now means trying really hard to avoid ever touching physical bottom with the tap. That seems to be the root of most evil. I would rather deepen the pilot hole a bit or decrease the thread count a smidge. There is some calculation out there that says anything more than X threads provides diminishing returns on holding power anyway.
I'll think more positively about this subject when I get this fr*ggen tap out