Roundup or hard graft?

Now that we've got the steps built up into the garden we've been working out how best to level off the not so level garden that the builders left us with. At the moment it's an undulating slope of assorted weeds with some grass mixed in. We want it to become a garden with borders of naturalistic planting (nothing too formal) with small areas of grass including grass/lawn paths. To achieve this we've asked a couple of landscape contractors to quote for levelling some of the slope to make into lawn areas.

The other areas that we'll eventually turn into planting borders leave us with an eco-conundrum. We either spend a lot of time and effort digging these over to remove the grass and weeds, or we spray them with a herbicide such as Roundup. The Landscape contractors advise on the latter and are quick to point out how benign a herbicide it is. We're yet to be convinced.

Just cos it's the world's biggest selling herbicide doesn't mean we should be happy to use it in our garden. However, if we dont use it the alternative means a lot of hard graft. Not that this is something we're adverse to, but worth considering the amount of fuel (both for power tools such as the mower and strimmer, along with food for us) that would be consumed as a result. What's worse for the environment? We certainly don't know the answer. So we'll ponder this a while longer whilst also waiting for the contractor's quotes to arrive.