If your goal is to lose weight, then get more time on the bike. You will burn more Calories per hour if you ride faster, but you burn more total Calories if you go for a longer ride.

More riding will also make you a stronger rider and build a base that you can use later for whatever events you ride.

But until you have an event in mind, whatever it may be, there's not much point to doing "training". Riding for fun will do as well, and it's more enjoyable.