Having done the SFH sharing thing, and graduated to buying MFs, I would probably never go back. If you want to build a big portfolio, this is the time to start professionalizing things - establish separation between you and your tenants on a personal basis, develop more formal processes for handling everything, etc.
Returns are almost always better on multifamilies so even if it means you have to start a little later, I'd go that route.
You might look into getting your current property reappraised and refinanced on a conventional loan if you have enough equity, which would allow you to go FHA again on a second MF. Plenty of other options too - from a strategic standpoint, a good mortgage broker might be able to help you identify and compare your options.