Are you looking to have your carpets cleaned, or go into business?
Truck mount is a bigger machine. So you get more power out of it. Truck mount doesn't affect chemical usage though... that just depends on what you choose.
Truck mount can use a self contained water tank, so you're not filling the portable tank by hand...
How far away the truck has to be parked can affect performance. And you may have to lug around a lot of hose, hehehe.
Either machine tho, will depend a lot on how well the person using it, knows their job.
I've used both over the years...if I were going into business, and had plenty of start up cash, I'd buy a truck mount. But I've cleaned a lot of carpets with portable machines too, quite successfully. Filling and emptying the tank on a portable is time that could be spend cleaning with a truck mount, so 'how often' the machine is used would also affect my choice. Right now, I only need a truck mount for flood recovery, heheh.