The issue of smoothness is based on design; timing and fields used. The way and where of polarity changes are based on a personal preference for DC.
I could just as well argue the cooling period between AC waves has a cooling advantage. Don't forget that DC voltage drop per foot and fancy switch wiring to reverse loop either.
There is only "best" for you and if it's Tuesday or Wednesday at the power company. Purple Hemi, Cross Ram, or Ford Hemi... now get it to "hook up".
The brushes wear is a bit into electrical theory with particals jumping with current, especially with the contstant direcction change of ac. I'll conscede to that, but I only need to clean or change brushes every decade or so anyhow. If it bothered me, a brushless motor would be my quest.
Occasionally you will get an open frame AC motor that does not like DC. It will run, but hot, jerky till you hit just the right speed. I found that out running postwar on car batteries at car shows, and on vacation in the woods, etc
..where rain is REALLY boring..
Any hot motor continuous motor may or may not, benefit from being run on half wave or rectified AC as during the ramp up and down of voltage waves the return of waves to 0v provides cooling time. Poles and fields balance, and timing is what decides smoothness.
That said, most of mine run smooth on DC.
The first time out on a battery I forgot about the whistle though and had to couple loco to coupler with wire as I didn't grab a non Whistler. It was a last minute thing, the stuff was still in the truck, new diesel & track, all I needed was some diodes for a throttle, (took ceramic resistors too) cars, a steamer, fishing pole, box, duffel and bedroll. That's best for a rained out Saturday
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