Well, SA-MP's player amount is sort of bigger than MTA. I want to point out something, SA-MP is up for a long time. And as based on my observations in the country I'm living in, most of kids (which are around 13 years-old) mostly playing SA-MP which don't even know what is MTA. There is just a lack of mature players playing SA-MP.  
 
And appearently (come and see) MTA brings way way more innovations than SA-MP, has mostly mature players and it's way more esthetic comparing to SA-MP. Although MTA is having ANY gamemode of SA-MP (additionally more variety + better developed gamemode types).  
Last of all, just as an example you can think MTA as a circle which contains SA-MP in itself.