another green house or go for red motel?

Looking for your opinion and any references to websites, books,etc.--

Wondering if I should buy another property (1-4 unit) or wait/save up and try to buy a smaller, 12-18 unit apartment complex.

Background-
I own a duplex and a four plex in the same area of town. It's ok neighborhood... not great, but not ghetto either. Been a landlord for about 5 years.

I want to keep growing my real estate business. I've read about sticking with duplex/single families vs. apartment buildings.... but I'm on the fence about which way to go.

Thoughts? Opinions? Questions I should consider?

Thanks in advance,
Todd