Hartman: It depends where you are in the cycle. In San Francisco, a very cyclical market, you have high risk, but at times during the cycle, you have very high rewards. So if you would have purchased during the dip and rode it up to the peak, which is probably near where we are now, you would have made a fortune. But it's not very reliable. In my many years in the business, I have never met anybody who could reliably predict the appreciation and depreciation cycles. If you can, then cyclical is best. But if you are not sure of that, then the linear or hybrid markets would be a much safer bet. If you are a more conservative investor, definitely, the linear markets.

Appreciation and depreciation cycles are very hard to predict, but cash flow or rental income is actually pretty reliable. You don't need a crystal ball to predict that people will pay the rent on properties most of the time. And if the rent-to-value ratio is good, then that makes those linear markets a nice, safe conservative bet.

GlobeSt.com: Why are real estate trends driving the renter population to increase?

Hartman: There are a couple of factors driving more rentals in 2018 and beyond. We have to look at Millennials, Baby Boomers, the Trump tax plan and rising interest rate impacts. As a macro trend, the trend for rental housing over the next 10 years is fantastic.

GlobeSt.com: Why are more Millennials renting versus buying?

Hartman: The largest demographic in American history is now the Millennial generation (80 million millennials versus 76 million Baby Boomers). And they are in a very unusual set of circumstances. Most of them have ridiculous student loan debt, which is like having a mortgage without a house. The other thing is, during their formative years, they saw their parents get burned in the Great Recession and struggle with housing. They are also very mobile, allowing them to move where jobs are located.

GlobeSt.com: What are the trends with Baby Boomers in real estate?

Hartman: At the other end of the spectrum, the Baby Boomers are now empty-nesters and may be renting by choice. This is a totally new trend that we've never really seen. In the past, Baby Boomers would always own homes, and maybe they'd buy a smaller home when the kids moved out, but now a lot of them are renting.

GlobeSt.com: Why will the ownership rate go down with Trump tax plan?

Hartman: For high-end markets, the new Trump tax plan encourages renting because mortgage interest over $750,000 in loan amount is no longer deductible. That will be an incentive to rent rather than own higher-end properties.

GlobeSt.com: What may happen to interest rates in the next six to 12 months?

Hartman: Interest rates will go up. And interest rate increases on the first phase will actually make the market improve because it creates urgency. And people who invest in a rental property will act with urgency and buy. But ultimately, it prices people out of the market and reduces the affordability index. And so it ultimately strengths the rental market.

GlobeSt.com: What will be the impact of rising interest rates on real estate?

Hartman: In the three dimensions of real estate, prices and interest rates are non-correlating indicators. So when rates goes up, it improves the rental market, but hurts the for-sale market because fewer people can afford to buy. So those two are opposite trends. And landlords and investors actually like high interest rates going up because they've already locked in rates in the past when rates were lower. And if rates go up in the future, it limits the amount of rental inventory that comes on the market because investors aren't buying as much. And the people who are renting can't afford to buy, so it puts upward pressure on rents. So landlords actually like higher interest rates.