Monoglot: From EO to next language.
I'm mostly a monoglot, but I live in Arizona and have many friends in Brazil so I have beginner skills in Portuguese and Spanish. I want to solidify and become proficient in Esperanto first, then circle back to Spanish/Port. Between those two, I guess it really comes down to my motivation. I've seen PT as a gateway to Spanish (since most word are 80% Spanish), and love it because it sounds much cooler with its french and Italian influence. However, in Arizona, I'm much more likely to run into a Spanish speaker. Decisions, decisions.
In the end, it comes down to motivation.
I'm just some stranger but to me, it sounds a lot like "I should learn Spanish because I meet many Spanish speakers". This sense of duty is not always the best advisor for learning a language - sometimes, we are just motivated by irrational things like "It sounds nice" and that's fine.
Focusing on Esperanto while you're still on a beginner level might really help to speed up your learning... There are studies that proved that Esperanto can be really beneficial for learning other languages!
Once you're familiar with either Portuguese or Spanish, I would suggest the Portuguese <-> Spanish trees for training BOTH languages and also memorizing the differences between them.