The best language if you want to go on to Spanish is Portuguese, and vice versa, with Italian also being helpful. Either Spanish or French (or both) are very helpful for learning Italian. I do not know or speak Latin, but people who do have told me that it's less related to the modern dialects than they are to each other. Church Latin and Classical Latin were already older dialects by the time the late vulgate tongues began to diverge from each other.

Learning Latin can be a reward in and of itself, of course.