Think I might get better life out of a larger diameter tool or more flutes? I’ll probably stick with the 4 flute 3/8 as I hope I’m almost done with this job and still have a couple spares.
I don't think the life of the tool would be any different once you have the parameters right.
You could put more power through a heavier tool without risking breaking it so you could probably still double the feedrate and it might even cut better. Of course you have to adjust the RPM to a new diameter. And you can also increase your depth of cut too - up to the point where your machine still has power or your setup is strained. Be careful doing both at the same time.
The critical number for tool heat/wear is always the feedrate through the material.
A long time ago, when I was a young engineer I learned how to set up speeds and feeds for all kinds of materials in a large aerospace factory. I became the go to guy for the stuff that was really hard to cut. The funny thing is it was a union shop so I couldn't touch the machines. I always said when I retire I was going to have a machine shop. I'm still working but I have a couple machines that I play with. Most of my knowledge is pretty outdated now.