That will become two different animals for you. Flowers usually stay still unless the wind is blowing. On flowers you can set up on a tripod and light from a different angle.
Insects move, they move a lot. You cannot usually count on a tripod shot and you won't be able to get real close using a hand held camera method.
Longer focal length for insects that fly so you have working room and prevent you from shadowing over your subject. Shorter focal length can work for flowers and stationary (slow moving) insects.
Also, are you wanting to do true 1:1 or greater macro shots or just good clean close-up shots?
Thsi is a macro shot using a reversed lens on a D90:
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This is a close up shot taken from a distance with a 70-300mm lens:
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