This is completely up to you. What sort of look are you going for?
In contintental Europe the Lever on Rose has been the more popular style, whilst UK designers have tended to favour the traditional levers on plate. More recently however, the British have started to take a liking to the more contemporary levers on rose. It all of course depends on the property that you are using them in, and the look that you are after.
If you are using the handles in a period property, then you will want to use only levers on plate.
If you want to go for a modern look, then levers on rose would be advisable, but you must bear in mind that there is no strict line. There are many contemporary designs available for levers on a plate (especially dual finish ones), if you prefer. This type of handles can be a good all-purpose handle if you're aiming for neither a classical feel nor an ultra-modern feel.