OK, here's my salsa/merengue quick test that will seldom fail you in deciding which style a song you are heariing is. Listen to the percussion. If it is merengue, the beat is steady, driving, unrelenting and energetic, almost bouncy or hoppy. You could stand in one place and just jump up and down and still be in the right timing. You can count a steady one-two-three-four, repeatedly and not get lost. Of course the songs have stops and starts, but for the most part, you can stay in time with that steady beat. Also the trumpets and brass have almost a staccato feel that matches the percussion.
In salsa, if you attempt to count a steady one-two-three-four, you will get lost. The pulse is way more complex. In fact, pulse is a good word, because the percussion in salsa is more pulsating. The brass is more melodic and the piano plays a prominent role, often soloing and employs typical salsa riffs (that I love, by the way.)
One other point to help you (I hope), salsa has developed two distinct branches. The traditional salsa is exemplified by Los Van Van, Ruben Blades, Celia Cruz, The Buena Vista Social Club and numerous others. The "New Salsa" is a pop derivative. Check out Marc Anthony, Son by Four (whom I actually do admire...they are more than just N'Sync salseros), even Jennifer Lopez...OK, she's a stretch. I'd call her "nueva salsa/dance."
For your purposes as a bass player attempting to increase the styles you play, you would do well to explore the traditional salsa which still has devotees in the millions and is the music that fills the salsa clubs; plus learn to play merengue because it has its ardent adherents too. Often those who dance to the one look down on those who dance to the other. If you want to lose weight, merengue dancing would do it the fastest, I believe.
As a bass player in merengue your challenge will be to keep up. In salsa, your challenge will be to get in that challenging groove or clave as it is known. When I lived in South America, musicians spoke so lovingly and reverentially of the clave, I came to believe it was something mystical and inborn...a quality extremely hard for a Yankee Gringo to capture. Indeed, I never quite have.