the cooker is on a flex due to the fact it can be moved to be cleaned or maintained.
a hob is a fixed appliance so needs fixing in a permanent position.
so rigid pipework and an isolator so it can be isolated :0
remember to check the site requirements of the hob .. manufacturers state usually 750 to a non combustible material ie: cooker hood.
some state 650 .. so check the manufacturers instructions before fitting the cupboards...
also check the distance either side of the burners to ..
quote from http://www.biasco.com/pbguides/hobs.php
IMPORTANT SAFETY ANNOUCEMENT
Care must be taken when installing gas hobs, as there are specific regulations that must be adhered to. There should be a minimum distance between a gas hob and the cooker hood or unit above of 750mm. If the hob is installed adjacent to a tall housing unit, then it should be at least 150mm from that unit. There should also be also be a gap on the other side of the hob of at least 750mm. If the hob is being installed in the gap between 2 tall units, then there must be a space of at least 300mm either side of the hob.
To avoid any confusion (and fires!), gas hobs should always be fitted by a C.O.R.G.I. registered fitter who will be familiar with these installation requirements.
It is illegal otherwise.