Hardware configuration: PI2, PI2 + PI3 or PI3 for HASS and DB?

Topic from reddit: https://www.reddit.com/r/homeassistant/comments/5mchvo/two_raspis_3_and_2_for_hass_and_database/

I’m currently running HASS on RasPi 2. I gather lots of data from zwave devices (i.e. power consumption) which I store in MySQL database. I later analyse them and make charts. I noticed that HASS is sometimes struggling to show my logbook or history. From time to time I do perform more or less complex queries on MySQL from the HASS data (that’s the only database on Pi2). I started considering moving some functions from Pi2 to another Pi I have - Pi3. This one is much snappier and I’ve never seen it not able to catch a breath. In my mind moving MySQL to Pi3 would be already a relief for Pi2. However, I would like to hear from your experience what is better - which machine in your opinion should run what - HASS on slower and MySQL on faster Pi or the other way around? Which service is more demanding at the end?
Configuration: - Pi2 + ZWave hub + WiFi + Bluetooth running HASS and MySQL; 5 ZWave devices (one of them power consumption sensor with data every 2-5 seconds) - Pi3 not doing much yet
Thank you for your recommendations!