Below you can find a list with advantages and disadvantages of buying Off-plan and Second Hand Properties.

Off-Plan Properties

Advantages
Investment Factor - If you purchase early in the construction phase or even before the construction has started the investment value of off-plan properties is very good. Initially the prices are set below market value to attract investments, which help the promoter finance the construction. Furthermore it is possible in many promotions to re-sell the property before completion and make a decent profit.
Early Purchase - If you purchase at an early stage it is also possible to choose the best properties i.e. with the best views, penthouse, corner etc.
The Seller - When you buy a property under construction you will buy directly from the promoter. Although you buy through an agent you only pay the price set buy the promoter and you therefore don't have to finance any agent fees.
Guarantee - New buildings include a 10 year guarantee for main failures in the structure, roof, etc..
Being New - You are the first to occupy the property and can therefore personalize it as you wish. You don't have to pay expensive changes to things the previous owner has installed.
Availability - The offer of off-plan properties is at the moment very good with a lot of different options available all over Spain.
Price - The prices for new promotions are set by professional developers that have a much greater knowledge of the market value (compared to private sellers) and you will therefore not find major variations in prices for off-plan properties of same standard. The difference in price is reflected by other factors such as the standard/quality of the development, the location, views. etc..

Disadvantages
Seeing the Property - When you buy off plan you will not be able to see the finished product (the property) until the completion of the construction.
Time - To be able to get a good and cheap property under construction you will have to buy early, which of course means that you will have to wait a long time before your property is delivered. (normally about 6 months to 3 years after the initial planning has started).
The Area - The area of the development can change - although you inspect the plot where the construction is going to be build you newer know how the area is going to change during and after the constructions (your lawyer should look at the local town plan for the area although new licenses can still be given after you decide to purchase).
Late Purchase - If you buy a property under construction in the final phase the investment value will be significant lower than an early purchase and the best properties will have been sold.
Delays - A high percentage of all new constructions in Spain are delayed either because of bad planning or because of some problems with getting the final licenses.
The best way to make sure this doesn’t happen is to buy from respected promoters and to make sure there is a penalty in the contract in case of delays.
Bankruptcy - The point many buyers fear is that the constructer goes bankrupt or that the qualities of the property are not according to the information given before the purchase. Again the best way to ensure the above doesn’t occur is only to buy from promoters / constructors that already have a good reputation and you should also make sure that a bank guarantee and insurance are in order. You should also assure that all the rest of the papers are in order such as building licenses etc. as these are the most frequent reasons why a construction is not completed.

Second Hand Properties

Advantages
Fast Buy - When you buy a second hand property the waiting time to complete the purchase is much less than when buying a property under construction. If the seller agrees it is possible to complete the purchase at the notary as soon as the buyer has the money ready and his lawyer has gone through all papers i.e. made sure that there are no outstanding debts on the property.
Negotiable Prices - Although the investment value on average is lower compared to buying off-plan in an early stage it is still possible to find a good buy and it is almost always possible to negotiate the price.
Clear Inspection - It is possible to visit the actual property (and more than once if necesarry) to make sure that it is the right property and to go through and inspect all details.
The Availability - The availability in second hand properties is more varied than off-plan developments. You can find everything from old properties (fincas, apartments, townhouses etc.) that needs complete renovation to furnished and ready to move into properties. Furthermore there are many areas (i.e. the city centre, the country etc.) where the only option is a second hand property as it is very difficult or impossible to find new developments because of lack of space or restrictions towards building.

Disadvantages

Finding the Right Property - As every second hand property is different it normally takes a long time and many visits to different properties is necessary to find the right one.
No Guarantee - There are no guarantees on second hand properties, which means that if there are any damages to the property after you have bought it you will be reliable to fix it.
Agent Fees - Second hand properties are normally sold through real estate agents. Although many agents provide a very good service their interference will in general increase the price of the property as the commission of the agent is normally put on top of the net price which the seller wants. (Officially and according to all agents the seller pays the fees, which is also technically right as the agents fees is already included in the sale price. The way it actually works at the moment is that the seller tells the agent what net price he wants out of the sale and thereafter the agent ads on his fees to get the final sales price).
Price - The prices on second hand properties are set directly by the private seller and there is a great variation in price from property to property even within the same standard. Although it is the real estate agents job to advice the seller about the price the seller have in many cases already decided upon the price before contacting the agent. As prices can vary a lot it can be recommend to see many different properties before deciding upon one or to seek independent advice from your lawyer or someone knowing the market.

Major differences of buying a property off-plan compared to a second hand property