People lump the two coasts together because both are sea and hills within a couple of hours of Barcelona. Renting in one is not the same exercise as renting in the other. One is a commuter coast with a market that runs all year. The other runs on summer, and the villa you can rent in August is often not the villa you can rent in January.
The Maresme starts at the edge of Barcelona and runs northeast to Blanes: Alella, Teià, Cabrils, Vilassar de Mar and Vilassar de Dalt, Premià, Sant Andreu de Llavaneres, Sant Vicenç de Montalt, Caldes d'Estrac, Arenys de Mar. It is a working coastline, with a train line, the C1, and towns that live here all year.
The Costa Brava begins roughly where the Maresme ends, past Blanes, and stretches up to the French border: Lloret, Tossa, Sant Feliu de Guíxols, Platja d'Aro, Palamós, Palafrugell, Begur, l'Escala, Roses, Cadaqués. It is a holiday coast first. That single difference explains almost everything else about renting there.
If you need a home for a year or more, and especially if you or your partner work in Barcelona, the Maresme is the practical choice. Long-term villa and house rentals exist here because there is year-round demand from commuters, families with school-age children and people relocating for work. You will find monthly contracts in Alella, Cabrils or Llavaneres that a landlord is happy to sign for five years.
On the Costa Brava the same search is harder. Much of the stock is built and priced for holiday weeks, and owners who can earn summer money often have little reason to sign a long lease. Long-term options do exist in the bigger towns, Blanes, Palamós, Palafrugell, and inland, but the choice is thinner and the good ones move quietly. Budget more time, and expect to hear "that one is only for the season" a lot.
The two coasts also differ in who you deal with. In the Maresme it is usually a resident owner or a local agency that manages long leases. On the Costa Brava you are more likely to meet an owner who is present in summer and unreachable in winter, which matters when the boiler breaks in November.
Choose the Maresme if Spain is your base, if someone in the household works in or near Barcelona, if you have children in school, or if you simply want neighbours in January. The trade-off is that you are not really on holiday: the coast is quieter than the city, but it is a place people live, not a resort.
Choose the Costa Brava if the plan is seasonal, a long summer, a sabbatical, or a holiday home you use part of the year. Renting a villa there for a few weeks is easy and the market is deep. Renting one as a permanent home is possible, but treat it as a project rather than a listing search, and start months before you want to move.
One last thing. On both coasts the best rentals, especially the ones at a fair price, tend to be handed over between people who already know each other, or through an agency that hears about them before they are published. Wherever you land, it is worth having someone local watching before the listing appears.