'Kingspan' polyisocyanurate has a thermal conductivity factor of about 0.022 whereas mineral wool fibre is approx 0.035 - 0.04... obviously, the lower the TC value the better.... 120 mm of kingspan would give the same insulation value as appro 300mm of quilted insulation...
however....
theres a few points to remember about the kingspan board though....
its a gaseous based foam, so when you cut it you release some of these gases (you can smell it when its cut).... this arguably reduces its performance....
its also usually foilbacked, but under the foil is brown paper.... the long term performance of this has also to be proven (its prone to rot if wet)...
also the cutting of the board has to be fairy accutrate, you will have to fill gaps with the expandable aerosol foam...
personally i would go for a breathable type insulation like sheeps wool (ok, its too expensive) or a hemp based product, or softboard (which is wood fibre based)...... im not entirely sure if rockwool is breathable, but it (supposibly) doesnt sag if wet, whereas mineral glass fibre does.... so rockwool is an option
http://www.uvalue.ie/index.php?categoryid=15
this product is also becoming popular, although AFAIK it hasnt been agrement certed yet..... im not offering any opinion on it....
remember, if you are installing insulation between rafters along a pitch, you need to leave about 50mm of an air gap to the underside of the felt.