The title says it all. I'm unsure whether to go for the realism of the language or the ease of the reader by using English .
If you use Spanish for the dialogue, the vast majority of readers won't be able to follow it. Any publisher or agent (real ones, not the frauds charging you fees) will read your novel cover to cover and see it. You will limit your audience to bilinguals, which will almost certainly be native Spanish speakers that also know English.
That is a small audience (relative to all people that buy English language novels, which includes many fluent in English as a second language).
"Realism" is not THAT important, you are taking the word in the wrong way and too literally. Realism is about portraying a realistic human experience; highly plausible emotions, motivations, reactions and choices, so the reader is not, in a literal sense, alienated by the story, it means not breaking the suspension of disbelief that lets readers relate to the heroes and side characters as if they were real people, and to the setting as something presenting "realistic" opportunities, obstacles and threats (even if it has fantastical elements).
I'd highly recommend you use English throughout, and if you present Spanish for flavor, find an excuse to translate it or make the meaning obvious.
P.S. As a reference point, there is a current television series, Queen of the South, in which the lead and all main characters are native Spanish speakers. Nearly all their dialogue, even with each other (which is frequent), even discussing crimes, love, sex, threatening to kill each other, is in English. If they speak in Spanish (once or twice in a show) there are always sub-titles to show what they are saying. I would try to accomplish the same thing in a novel. It may not always be so, but at this point in history English is the most understood language in the world, and therefore your biggest audience. That's why the professionals do it this way.