This brings up another question. Since everyone loves end mill holders, and people tell me that 3/8" is often the best size for HSM toolpaths, why don't 3/8" endmills come with flats? 1/2" sometimes do depending on the manufacturer (the MariTool/HTC ones do), but I've never seen one stock on a 3/8".
I just spent a few buck on a "green-wheel" and lick a small flat on by hand when i want to really lean on a tool in a side-lock.
I really need a diamond-wheel though, as that green-wheel is bouncy, and wearing quick.
Regardless, my spindles live @ 12k RPM, and I can not feel a difference in vibes between a collet, and home-brew flat in "good" side-lock.
No, I do not have a balancer. But, I do have good ears, and sensitive touch, LOL. And, my spindles seem to last a long time.
Even the one dip-shit crashed, that I thought would be dead a year ago, is still going.
BUT! For a 3/8" tool? I seriously doubt you will see a benefit going to a side-lock from an ER32 or TG75. 3/8" is tiny.
I go as fast as a HAAS super-speed will go with 3/8" S-carbs daily (2-300ipm all day long) in ER32, and they don"t make a peep, and last seemingly forever.
As a matter of fact, the S-carb I am running right now has filled over 40 55gal drums with 6061 chips, and still going strong, in an ER32.