Thanks for all the answers, yes you are maybe right that it doesn't matter much which method I use, but there will be alot more in the loop than the clock code. I will use capacitive sensors (link) as buttons and play some melody on a piezo element when the alarm goes off etc.
I think I'll try with divisions and modulo first and rewrite if there is problems.
@RuggedCircuits, Thanks, it's always good to know how it is, if I need to, I know what i could try to optimize.
@Kitep, yeah it would be intresting to see how big the diffrence is, maybe I should do a test.