Many early investors to Morocco are looking for answers to this question.
Abby Aron author of the book 'Buying a House in Morocco' has kindly let me re publish her article and thoughts on this subject

One of the questions I am most commonly asked is, which of Morocco’s two coastlines, the Atlantic or the Mediterranean, will offer the best rental returns?

The best way of answering this question is by looking at what it is that potential renters want out of a property when visiting Morocco.

At the top of the list is climate and on an equal footing, location. By location, I mean proximity to an airport or ferry hub, proximity to the beach, proximity to leisure facilities such as golf and proximity to attractions.

When it comes to coastal holidays, Agadir on the south Atlantic coast, still tops the charts for popularity according to bed nights recorded by the Moroccan Tourist Board for early 2007, four times more popular than Casablanca and 6 times more popular than Tangier.

This is because Agadir benefits from having an international airport with daily flights, year round sun, beaches, three golf courses and closeness to attractions such as the rose red city of Taroudannt.

Elsewhere on the Atlantic coast, you have Essaouira, which will have an international airport by 2010 and benefits from an old, picturesque medina, music and art events, which are always a draw. Cities around Casablanca, such as El Jadida, although still relatively untested in terms of the western tourism market, are expected to start drawing large numbers with the luxury upmarket King’s resort coming soon and, of course, proximity to the hub airport of Casablanca with good new motorways making day trips to the very popular, Marrakech a distinct possibility.

With the government’s surge to quadruple tourism numbers to Morocco by 2010, the rental returns along the Med are likely to increase dramatically.

When the regular flights begin, Tangier and Tetouan, will be able to offer similar rental returns to Agadir due to having their own international airport, beaches, culture and a number of top quality golf courses not to mention the promised luxury marinas. The flagship King’s resort at Saidia close to Oujda is obviously going to have its own marketing machinery for maximising rental incomes but these are likely to work in isolation from the surrounding Mediterranean coastline.

Climate wise, the Atlantic benefits from almost year round sun and a temperate climate although bizarre things have happened recently like snow in Casablanca.

The Med is rainier in winter months but hotter in the spring/summer. The Atlantic sea for swimming is less reliable than the Med due to cooler temperatures and larger waves.

So to sum up, best rental returns will most likely come from properties on the Atlantic coast quite simply because here, you have the diversity of the regions.

The Med coast, however, benefits from a warmer wind-free climate, calmer seas and proximity to southern Spain, which will appeal to holidaymakers on weekend breaks.

The best way to approach this question, therefore, is to obtain all the facts. What is up coming in terms of infrastructure and facilities, where are other people buying? Then ask yourself what most appeals to you and why and you can be pretty sure if it appeals to you, it will appeal to many others out there too.