Before going deeper on my opinions there needs to be some grounds rules, such as how each league was founded.
The MLS or Major Soccer League was founded because of the United States being awarded the World Cup in 1994. Prior to this event, the top division of soccer in the United States was the Major League Professional Soccer League (now defunct). In the inaugural season it had 10 teams, with both conferences being comprised of five teams each. As is the case of soccer in the United States, after the World Cup there is a bubble and then a big decline in viewership, the MLS struggled harshly post 1994, 1998, 2002, 2006,2010,2014. In 2007, the MLS created the Designated Player Rule, which allowed for soccer stars like Andrea Pirlo, Kaka, David Beckham to be paid above what other players made to attract stars to the league. Currently, there is no promotion and relegation system in the MLS and there are 23 teams (11 for the Eastern Conference and 12 for the Western Conference) with two more teams joining in 2020.
Futbol de Primera Division
The Costa Rican Domestic League was founded in 1921 with seven teams. Only three of them are still active: Alajuelense, Herediano, and Cartagines. The league was formed in conjunction with a national team on the same year for the Juegos del Centenario in Guatemala (Referring to the event where all of Central America but Belize and Panama gained in 1821 from Spain). Ever since then, the league grew to twelve teams and then the Unión de Clubes del Fútbol de la Primera Division or UNAFUT was made in 1999 with the National Team Federation oversees all aspects of soccer in Costa Rica except for LINAFA (the third division and lower divisions).
Arguments for the MLS
The MLS has the superior players: Zlatan Ibrahimovic or Thierry Henry or Kaka. It also has more possibilities to go to the CONCACAF Champions League as it has automatic bids into the second round of the tournament. Although the league is relatively young, the level of play is as high as the Liga MX (Mexico’s league and the most dominant of the region). In addition, there hasn’t been an overall dominant team as the team with the most cups is the LA Galaxy with 5.
Arguments for the Futbol de Primera Division
It is much older and as a result, has many memorable rivalries like the Clásico Nacional and the Classicos Provinciales. The teams who make it to the CONCACAF Champions League have done relatively well even winning it all with Alajuelense, Saprissa and Cartagines winning at least one. Most of the National Team call ups in the first ever Aztecazo were playing in the league before defeating Mexico in the Azteca (a feat that the United States hasn’t done it in World Cup Qualifiers). There are two tournaments of 22 fixtures for a total of 44 matches per team year round, as is the trend in Central America and also Mexico. Even though the stadiums are relatively smaller, the biggest being the National Stadium having around 30,000, most of them sell out for important matches. There are also not just one but two important races to keep an eye on, the top four and the “No Relegation” which could have as many as 5 teams.
My Verdict (email me at firstname.lastname@example.org if you disagree): Putting my biases aside, I have to go with the Costa Rican Domestic League. While the MLS has gotten better the last couple of years, the league hasn’t been as strong other leagues in this stage of infancy. In addition, soccer leagues in general function as the worst gets relegated and the best from the other division gets promoted to counter the usual matchups. Moreover, the fact that team who generally sucks gets the first overall pick in the following draft makes no sense in this sport. Before the Designated Player Rule, MLS viewership was low and it has an undulating interest level where the high is when the World Cup is that summer. In addition to the Promotion/Relegation problem in the United States, there has never been any Champion from the United States or Canada to win the Champions League whereas Costa Rica has three teams with a title or two.