I like microSD cards. No trusting your data to file hosting services. No being dependent on having access to Wi-Fi or having to worry about burning through your cellular data allotment. You know your files are accessible when you want them.
When you're looking to buy a microSD card, one aspect to consider is whether it's big. Small is fine, but really, what you want is big. 32GB holds more music than 16GB. 64GB puts 32GB to shame. 128GB crushes them both.
Samsung makes a good class 10 microSD card with up to 128GB of storage that will make pretty much any phone with expandable storage happy. Now you can get it for $39.99.
We saw the card drop down to this price last year, but the cost went back up. That could happen again, so if you like sticking a card in your phone with more space than you probably need, you know what to do.