I’ve used every size pump as well from 15lt/min to the 40lt/min I use now. I would never go back to using anything less than 40lt/min. But it has its drawbacks, you need a big water tank in the van so you need a bigger van. bigger machine clean quicker they run a FSC more effectively and rinse down quicker
But most peoples opinion of the best machine is often (coincidently) the machine they use, hence my view that 40lt/min is best, if I use a 21lt/min machine I bet I would be singing the praise of that set up