Investors trying to maximize the income their portfolios generate often get lured in by the prospect of giant yields. That makes sense, since a higher yield will mean higher income -- at least in the near term. The problem is that some high-yielding payouts just can't be relied upon to continue. That's why investors looking at mortgage real estate investment trust (REIT) AGNC Investment's (AGNC 0.58%) huge 14% yield might be better off with the relatively modest 5% yield of standard REIT Realty Income (O 0.58%).

The problem you shouldn't ignore

There's no question that an investment in AGNC today will produce more dividend income in the short run than the same investment in Realty Income. The dividends you would collect would be substantially higher, given the difference in their yields. But here's the rub: Over the past decade, AGNC's actual payouts have been heading steadily lower while Realty Income's payouts have been heading steadily higher.

AGNC Dividend Per Share (Quarterly) Chart

AGNC Dividend Per Share (Quarterly) data by YCharts

For investors who want to use the income their portfolios generate to cover living expenses, that chart should be eye-opening. Imagine making an investment under the premise that it would pay you a certain amount of money each quarter, only to see those payouts decline by 55% over the course of a decade. On top of that, AGNC stock's just so happened to track its payout lower, so you'd have less income and a roughly 50% drop in the value of the investment. That's a big capital loss on top of a big income shortfall. 

By comparison, Realty Income increased its dividend by 39% over the last 10 years through regular annual increases. And the stock's price increased by just about the same amount. That's a win/win scenario. Notably, by the end of that decade, Realty Income's dividend was actually twice the size of AGNC's dividend.

Solid as a rock

The key difference here lies in what these two REITs actually do. AGNC is a mortgage REIT. That's a complex business model that involves buying a portfolio of mortgage securities. Only investors who have the time and energy to do a deep dive into the mortgage space should even consider these types of REITs. Realty Income, on the other hand, owns physical properties. Kitchen table finance is enough to understand how that business works -- buy property, lease it out. 

On top of that, Realty Income uses the net lease approach. It rents properties to single tenants that are responsible for most property-level operating costs. Although there may be some risk that any single property's tenant won't pay its rent, across the REIT's portfolio of 12,000-plus properties, the risk is modest. And because as a landlord, it delegates the day-to-day operating issues to its tenants, the business is super simple. 

Then there's the REIT's balance sheet. Realty Income has earned an investment-grade credit rating, which is a sign of financial strength. It also means that the company should have relatively easy access to capital markets at relatively attractive rates if it needs cash. Note, too, that Realty Income is the largest player in the net lease space, with a market cap of more than $40 billion. That should allow it to more easily issue stock if it chooses to, and also means that Realty Income can ink deals that some of its peers would be too small to handle.

Put it all together and Realty Income is one of the most boring and reliable REITs an investor can own. Indeed, it has increased its payouts annually for nearly three decades. That annualized rate of increase over that span was a solid 4.4%. That's a touch higher than the historical rate of inflation growth, too, so the buying power of its dividend has actually increased over time.

Go the reliable route

For the vast majority of dividend investors, Realty Income will probably be a better choice than AGNC. That's not to suggest that AGNC is a bad mortgage REIT, but if reliable dividend income is your real goal, it just hasn't lived up to expectations. Realty Income, on the other hand, is the model of reliability -- which is probably why it trademarked the phrase "The Monthly Dividend Company."