Probably an Asian language would work with either of those two -- as they are sufficiently different. I would suggest Mandarin if you are interested. It is not a hard language grammatically as the verb form stays the same for all persons and doesn't change with tenses either. The hardest part about Mandarin for me is that there are so few cognates and the ones that there are don't sound that much like English to me to be instantly recognizable.

My best tip for learning any language is to play/listen to songs in the target language. You don't even have to pay attention to the lyrics get the benefit, so play them in the background and even while you are sleeping. When I was a pre-teen my mother was constantly playing music all day long by a French singer she loves, Charles Aznavour. My siblings and I used to run outside to get away from the music! But about two years later when I was struggling to make C's in French class (my mom forced me to take it) something clicked one day and from then on -- straight A's in French. Then as an adult learning Mandarin I would play Chinese children's songs for my kids in the car and something similar happened -- my Mandarin speaking was very choppy and hesitant even when reading from printed text. But again one day something clicked and the words were just flowing effortlessly.

Good luck with your language learning!