It really comes down to your personal data consumption. You can, however, monitor your data allowance by downloading the GServices app from the Google Play Store or App Store.
I've been having the same problem and have been talking to Globe via Twitter. First I was using Gservices. Then I found out that its data meter is not reliable. So Globe advised me to use My Data Manager. I did. And just today, I proved that Globe really uses the free mobile data first before consuming the free Spotify data. I have all the screenshots to prove na hindi totoo yung claim na gagamitin daw muna yung Spotify data if you're using Spotify. I'm subscribing to GS10 because that's supposed to be sufficient for my needs. It was perfect for me. I only use Viber (which is free), Facebook (free also), Waze and Spotify. The 40mb allocation was just perfect. But sad to say, when I listen to Spotify, inuubos ng Globe yung 40mb allocation ko. Hay, hay, hay. Please be truthful sana with your claims.