I agree with previous answers, but to add:
Investing is about making money, either through appreciation or cashflow. Complexes come with land and condos don't, so complexes are likely to appreciate better.
However, investors in residential property usually focus on cashflow, and I recommend you do too. Condo's do come with HOA fees, and complexes come with common area utility and maintenance fees that the owner usually covers. But apples-to-apples, each individual complex unit will usually be more cashflow positive than an equivalent condo unit, so I always recommend complexes over condos.
Regarding how long to hold - hold as long as your cashflow is good. Most owners I work with hold on to a rental complex until maintenance expenses due to aging begin to dig into profits. For example, if you have a property that will need a new roof in 5 years, you may want to sell it now while it's still looking good and let the next owner bear the expense of the new roof. The same could apply to mass retrofit of aging appliances, aging wiring, aging plumbing, a cracked driveway, a pool that needs resurfacing, or any other major expenses that can be forecast.
Another advantage of complexes is that they can be torn down. Of course this will dig into your cashflow for awhile, but the investment strategy is that you may be able to sell the property for a higher price later based upon the new owner being able to replace your aging 18 unit property with a new and more profitable 24 unit property - if zoning supports it. You may even want to tear down and re-build the property yourself. Owning the land gives you options.
Best of luck with your investments!