This is where it gets crazy, and debatable at the same time. Do a quick search on here to read up on LEDs you will save yourself a headache, but a quick sum of it all is. Some people believe that you need to provide as much of the natural sunlight spectrum as possible to bring out colors, alot of people are even adding "Violet" LED's to enhance coloration. THe typical blue and white will grow corals and it will look good. THe problem that people argue is that some corals need a certain spectral input for colors. THis is also where it gets really scientific. For photosynthesis your blue and white put out more then enough light in the 430nM to 460nM range, but the problem is even with just the whites there is very little of any other spectrum. LED's are pretty much spectrum specific. I say again do a little reading, ask a few questions as you go, and plan this out yourself. If you were to go and only do whites and blues your COrals will still grow and be happy, coloration of certain corals may be different then someone that has several different colors. Not better or worse just different.
You will need the 3w LED's though, the 3w LED's pack more of a punch to get through the column of water. I think that I read somewhere the water column can filter and refract up to 80 percent of your usefull light. SO obviously making sure your water is clear will aid in this, but there are still minerals in the water column that will stop the light. THat is why the 3w leds are used.
As far as going with Cree's though I have built several systems with Cree, I am a fan of the Phillips Luxeon ES series. Half the price and just as good. Yes Cree's have more lumens at certain ranges but ask how many people have their LEDs ran at full.
Phillips luxeon ES : Neutral White @350mA has 130lmns ($2.99 ea multiplied by 144 = $430.56
Cree : Neutral White (depending on bin but I gave the highest (RJ) @ 350mA has 122 lmns. (5.00 ea multiplied by 144 = $720.00)
(waiting for someone to bring up how the bin number is important)
So the choice of LEDs is yours and this is another one of those debatable subjects, but I say after you do some research on your own, go with what you believe to be the best for you, your tank, and your pocket.