Which Tyres?

Right how big a difference do tyre weights make?

I'm specifically looking at tyres like rivendell jack browns, which come in green or blue, the former being 350, latter being 480g

They're for riding on all types of roads and paths but will be getting a lot of road use at first,

I don't ride to race but I would like to up my average speed for the 15 mark towards 20

I would says not much, it's a massive difference if you're riding says the Marathon Plus, as long the tyres is flexible and able to absorb the road vibration well, you'll be fine, get the former as bigger tyres is already less prone to puncture due to lower pressure and larger surface area than higher pressure tyres of the same thickness.

The latter (480g) will be less flexible, and may not be as comfortable as the former.