I looked into this before upgrading to a t/m.
The expence and complications of running an electrically powered rig compared to buying a truckmounted machine were minimal.
Being able to offer a completely mobile cleaning solution not dependant on electric or hot water being supplied was crucial to my work (lots of end of tenancy cleans).
If you need to work quickly and give good results a t/m is perfect.
I agree you can give as good a result using a porty, it just takes a lot longer!
My advice is: If you've got the work to justify a t/m, go for it!!!
All te best,
Jas.