Before you can decide what tools you need, you should define the job you are going to do.
If your potential market includes high rise condos / apartments and such, it will be easier to get a portable onto upper floors than to run hose up the side of a building. Will you have opportunity to clean buildings that want to lock you in for security - banks, big box stores, etc? If so, the portable is again the answer. Retail stores in a mall often require runs of several hundred feet of hose to reach from the parking lot. That can strain a used, entry level truck mount. So again a portable would do fine.
As Amy commented, middle income and upper income residential customers will likely expect a truckmounted unit if they have had their carpets cleaned with one before. A portable won't always convey the image you want for those clients.
If you would compete based on the lowest price, a portable could be a plus. You could market "No expensive overhead. We get your carpet clean for less."
Anyone who tells you what equipment to purchase without knowing about your business is really just saying what they prefer in their situation.
Look at your market. What areas have the stiffest competition? Which segments might be under-served? Also think about the schedule you want. Commercial work is usually nights and week-ends but it might repeat every month or every three months and be more steady year round.
If you have not thought about a business plan and a marketing strategy, start with that before deciding what equipment you need. What would you like your business to be like after 2 years? 5 years? 10 Years? Don't try to be everything to everyone. That does not get you toward whatever your goals may be.
When you have that nailed down, looked for a supplier who can show you several brands of new and used truckmounts and portables rather than someone who just wants to sell you whatever they happen to have. With 72 stores around the country, there is probably an Interlink Supply distributor somewhere close by.