If you fill all available space with the +1 mining structures (don't build artifact structures on either one), the Very Large planet will give you 28.5 resource points per hour when you first colonize it and 57 points per hour when it peaks after five days (plus any race/class research bonus that you might have). The Very Massive planet will give you 17.25 points per hour to start and 51.75 per hour after it peaks (plus bonuses). Normally, you want the biggest planets you can get simply because there is room for more structures, but the bonus is so much higher on the Very Large planet (100% compared to 50%) in this case and there's not a huge difference in size (57 and 69 building space) so the Very Large will give you more research points over time. They are both decent, but if you have to pick one I'd start with the Very Large...
edit: Agross beat me to it..
