Personally thumbs always..
most recently its best for me as I use the 2 sliders on the back of m f8g as idle raiser and engine kill... so it would be a hastle to use them if I was pinching..

I also switch between my 3 rates a LOT during a flight, even mid manouver sometimes...... and that I find best done with one finger all the time on the switch.. which is more comfortable and faster using thumb style....

But like many have said.. Its just a case of what you find "natural" and what works best for you. There is no right or wrong way... The same as I think everybody in my club has their rates switch in a different place... and they are all the right place, for them!