i have used both and have totally fallen in love with vallejo paints because of there ease of use,because there already thinned(model air paints) you only have to put as much as your going to use into the bowl.
i still like tamiya but i tend to only brush paint with those now and when there gone i will replace them with vallejo model colour.
i have a couple "rules" though...
1) always use a primer(i use tamiya fine grey)as they dont "grip" onto plastic that well without one.
2) i nearly always use the vallejo retarder when spraying,especially in hot weather.
3) let the paints dry totally before masking another colour etc.
any more questions ask away.
they really are very good.
paul
p.s. there matt,satin and gloss spray cans work really well.