Tyre choices - what's the extra weight provide?

I do a far amount of commuting and recreational riding which includes some light off-road riding. I
also ride a lot of rough country roads that can be more 'off-road' than off-road, particularly deep,
tarmac edged potholes along with surfaces mangled by agricultural machinery.

I need a tyre that will be ok for these conditions, fairly robust, upto tackling potholey roads
whilst carrying luggage but that's also ok for commuting.

There are lots of reasonable semi-slicks around but Wiggle currently has a deal on a couple of
Vredestein tyres that look to be promising. The first, the Vredestein G - Ribb, is a tyre I've heard
good things about both in terms of robustness and grip -
http://www.wiggle.co.uk/v2_product_detail.asp?ProdID=5300004078

The second, the Scarab, is more of a slick tyre but also look good -
http://www.wiggle.co.uk/v2_product_detail.asp?ProdID=5300006043

My question is this, both are 26x1.75, have PRS puncture protection and have semi-slick treads
but whilst the Scarab weighs 630g per tyre the G-Ribb is a sizable 800g, what does the extra
weight provide?

I'm not weight obsessed but equally don't want to go heavier than I need to.

So, is the G-Ribb a more robust tyre. I would also be interested in hearing peoples' views on these
tyres and suggestions for any other tyres that might be better suited.

Thanks in advance for any thoughts.

Kind Regards, Paul.