Which one is better? Sicilian or Spanish?

Depends on your goal.

1) Learning to play chess - Spanish, by a long shot. Spanish/Ruy Lopez has an attitude that tells the opponent "I will not back off by an inch". You basically fight for every square that is available, grab as much space as your opponents allows, construct the strongest center possible etc. Sicilian, on the other hand, says "do your land grab, I am concerned only with a narrow path to mount a counter-attack". Basically decision to play 1...e5 exclusively for a year propelled me from 1800 to 1900 Elo.

2) Beating a weaker player - Sicilian. I play 1...e5 from time to time, but I am careful not to try it against slightly weaker junior players who can prepare some boring stuff like four knights Scotch. But if such an opponent plays main line Spanish, then I may gladly go for it.

3) Beating a stronger player - I'd say Spanish, but it depends on your overall playing style. Basically days when a 2000 Elo players knew Spanish by heart are over now.

Now the funny thing is, my games after 1...e5 tend to be longer. There's some early pressure to survive, and counterattacks take slower to prepare.

"Sicilian" and "Spanish" both comprise a wide array of positions, with many different Pawn structures and many different plans. As mentioned above, a beginner-to-intermediate player would get more benefit out of learning typical Spanish openings... or even something strategically simpler like the Guioco Piano.

sicilian

Both are good. At the last World Championship Match Carlsen played Sicilian exclusively, while Caruana played Petrov, so no Ruy Lopez. At the last Yekaterinburg Candidates' Sicilian scored better for black than Ruy Lopez. So probably Sicilian > Ruy Lopez.

You should obviously play both, but at your rating, Spanish > Sicilian. 

The Sicilian is going to start getting pretty hairy.  The Sicilian is pretty sharp, so you have to rote memorize lines, or at least, have a strong idea of what's going on. There's also tons of traps and it's pretty easy for your opponent to transpose a Sicilian. So you may start the game thinking you are going to be playing a dragon, but you may end up getting something different, and if you don't know the traps, it can be an easy loss. 

The Spanish also has tons of lines, traps and tricks, but in general, you usually get what you want, especially if you use an exchange variation.