Two-thirds of the 25 analysts who follow Public Storage have "hold" or "sell" recommendations on the stock, which has returned 58 percent since April 2010, according to data compiled by Bloomberg.

Storage wasn't always so attractive to investors. In the five years through 2006, when the Bloomberg REIT index more than doubled, regional malls and shopping centers topped the ranking. Storage, while second by total return in that period, fell to third when adjusted for risk, because it had the second-highest volatility, after hotels.

'Low Barriers'

Those price swings coincided with a period where the supply of storage units increased in the U.S. New construction of facilities rose by more than half in the 2000s, with the fastest growth in the beginning and middle of the decade. The U.S. had an estimated 50,048 self-storage facilities last year, up from 29,955 in 1999, according to the Self-Storage Almanac, published by Phoenix-based MiniCo Insurance Agency LLC, which provides insurance and publications for the industry. Storage facilities also got larger, growing to an average of 566 units each in 2011, from an average 243 units in 2000, according to the Self- Storage Almanac.

"During 2001 to 2007, there was a great amount of new supply built because of low barriers to entry and cheap financing," said Teng of Public Storage. "All that has virtually come to a halt."

The relatively low capital needs of the storage business became more attractive after the financial crisis, as investors shunned companies with large debt burdens. Storage REITs topped the riskless return ranking since the end of 2009, with the second-lowest volatility and the second-highest total return. Regional malls, No. 2 over that period, had the best total return and the third-highest volatility.

'No Carpeting'

Storage units are relatively cheap to build and "when we re-rent a space, all we have to do is sweep it out," said Teng. "We don't have to change the carpeting, paint the walls" or otherwise make improvements to get a new tenant.

High leverage remains a concern for some hotel REITs, which have trailed in returns because recreational travel hasn't fully rebounded from the slump caused by the recession in 2008 and 2009. Hotel operators tend to see bigger swings in net operating income than other REITs, reflecting their lower operating margins, according to Green Street.

Hotel REITs returned just 0.8 percent over the past 10 years when adjusting for risk. They had the second-highest volatility and the second-lowest return. Office REITs, whose assets include well-known "trophy" properties such as the General Motors Building in Manhattan and Embarcadero Center in San Francisco, had the fourth-worst risk-adjusted return in the period.

Appealing Exteriors

Increased usage of Internet marketing has helped storage REITs attract more customers from smaller operators during the sluggish economic recovery, said John Murphy, a vice president at Cohen & Steers Inc., a New York-based investor in real estate shares that manages almost $45 billion. The storage business is fragmented, with the publicly traded REITs accounting for just 10 percent of the U.S. market, he said.