According to Terry Allen

Calls actually provide better downside protection than puts (as long as they are in the money), a special report:

Calendar spread using puts: Using the CBOE option calculator, with QQQ at $34 and an implied volatility of 20, the following option prices would exist. In this example, an at-the-money calendar put spread is purchased, and 30 days later, the price of QQQ fell by $1.00. The original spread purchased at $1.12 would be worth $1.27, for a gain of $.15, or 11.8% on the original investment before commissions.

Months to expiration 6 5
Price of QQQ $34 $33

Longer-term 34 put purchased $1.89 $2.27
One-month 34 put sold .77 $1.00

Net value $1.12 $1.27
gain: $.15 (11.8%)

Calendar spread using in-the-money calls: Using the CBOE option calculator again, with QQQ at $34, an in-the-money calendar call spread is purchased, and 30 days later, the price of QQQ fell by $1.00, just as in the above example with a put spread. The resulting percentage gain is a whopping 5 times the gain of the at-the-money put spread.

Months to expiration 6 5
Price of QQQ $34 $33

Longer-term 33 call purchased $2.43 $1.67
One-month 33 call sold $1.37 0

Net value $1.06 $1.67
gain: $.61 (57.5%)

The call spread continues to win if QQQ fell more than $1.00: If QQQ falls to $32, the 34 put spread would only be worth $.88, causing a loss of $.26, while the 33 call spread would be worth $1.22, for a profit of $.15 over the original cost. Once again, the in-the-money call spread provides better downside protection than the at-the-money put spread. (If we had started with an in-the-money put spread, the downside advantage for the in-the-money call spread would be even greater).

Of course, if you sold only 7 next-month puts for each 10 six-month puts you bought, the numbers would be different, but so would the profit if the stock remained the same. As with all option spreads, every adjustment has both a positive and negative potential effect, depending on the behavior of the stock.