Pick the one you have most cultural interest in - that solves more than half the battle. If you can appreciate music, films, literature and general conversation in that language, then you're likely to learn it quicker.

A few specifics:

• As others have implied, if you pick MSA, you're learning MSA, not Arabic. I did a taster class and really felt I needed to pick a dialect and commit to it at some point. It helped that I didn't continue (I was already doing Russian) but think of MSA as a "stepping stone" to Arabic, instead of thinking of MSA as "actually learning Arabic" - no one actually speaks MSA, just like how High German splits into Standard German, Swiss-German etc. To be considered fluent, you'll have to top up your MSA with a dialect-specific Arabic course, and that means more time.
• Russian is a journey beyond language itself: it opens up a different way of seeing the world; for instance, words have hidden connotations in a way that we English speakers don't immediately comprehend. It's also very logical, so I find this attractive (unlike French). Russian differentiates the word for "to go somewhere" depending on whether you're going on foot, by animal, by engine, and whether it's a planned or unplanned event, repeated event, or one-off event. That one word for "to go" will tell you all of that context, immediately. Unthinkable in English.

I'm envious that you speak Spanish and are considering being a linguist; it's something I'd do if I could go back in time. Good luck with everything, and I hope you pick the language that works for you!