well it is hard to measure, especially as ionic bonds aren't single, but instead as you say form a gaint ionic lattice. for gcse's you don't need to know which is stronger, just know that both are strong. The quoted figure though are, (f**k knows how they actually manage to work it out): ionic 120-450 KJ/mol covalent 120-550 KJ/mol however it varies greatly according to the crystal type.