Most CO-OPS have very strict policies about subletting, which does not make them an ideal investment opportunity and can present a serious challenge if your job suddenly relocates to London, for instance.  The rules vary, but owners are usually allowed to sublet their apartment for no longer than 1 to 2 years in any 5-7 year period. The board also gets to approve your tenant and charge you a fee for subletting.

CONDO sublet policies are far more liberal.  While there may be rules against short-term sublets (say, less than 6 months), there is usually no outside limit nor do boards have the right to turn down a tenant unless they exercise that right of first refusal and lease your apartment themselves. This makes condos ideal if you are looking to buy strictly for investment purposes [2] and rent out your apartment year round. But just like a co-op, the application fees, move-in fees, processing fees, etc., can range from a few hundred to a couple of thousand dollars extra that you or your potential tenant will have to pay. And, if you do make a home in your condo, you will be living in a building with a more transient population than a co-op. 

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Want to buy an investment apartment to rent out? Here's what you need to know [2]

The pied-a-terre checklist: What to consider before buying a part-time place in NYC  [3]» [4]