Most schools I have attended use decimal and it is the best method IMHO.
It makes adding up much easier and helps to avoid making silly mathematically errors. With using hours & mins if your having a bad day you can easily add them up wrong unless you have a fancy calculator that has hours, mins & secs function. To help you work out the decimal conversions you can always do a little card and keep it in your logbook:-
5 min = 0.1
10 min = 0.17
15 min = 0.25
etc etc. After a while you will remember them more than likely anyway but it will give you a handy reference whilst starting out.