For me it was simply what I could or couldn't afford. I started with a single family town-home. I wouldn't have been able to afford a multi-unit property. I don't feel with comfortable with all the alternative financing people discuss and went with a mortgage on the home. It was a great call and I was able to save the income from that property (while working full time also so that yielded additional savings and cushioning) to buy the next one and over a few years worked up to four homes, three of which are rentals (one a two unit).
The other thing to consider is if you are just learning and dipping your feet in the water, you will make mistakes. Even after doing it for years you will still make mistakes. If its a smaller investment, your mistake may be smaller and manageable. I'd rather make a $10,000 mistake than a $80,000. I could also bounce back from the former but probably not as easily from the latter.
At the the end of the day, you will receive a lot of advice and you have to decide what works for your and your budget. This was just the place I came from starting out.