i think this situation can depend on a number of factors, a lot of people on here have had a Seat touch up and it's been shocking, more like a match for luna grey and have gone halfords, had it mixed and it's been spot on.
Looking at the above its visa versa though, i'd be prepared to do both though if your wanting a good finish and one doesn't work. Don't use the brush you get with these kits though, thats one thing that applies to them all. Use a 0000 or 00000 model brush. £2.50 of the bay but worth it