well that all depends, first off you would not use LED's buy themselves, therefore using a mixed spetrum LED board is not the most beneficial. LED boards are used as supplemental lighting, therfore this means that you must have some other form of light, mostly being HO FLO's.
HO Flo's are not totally 6500K, that is just the spectrum that comes most from the bulb. You are still getting Some white, yellow and red. Therefore, i myself IMO, think that using an all blue board would be more benifical due to the characteristics Blue LED boards gives to the plant, which are branching and foliage. If you have an abundance of blue spectrum your internode length should in theory be shorter(which should in theroy let you veg longer with many more nodes). For example, side by side test with mixed spectrum and blue spectrum both with flos, should in theory either be the same height but the blue spectrum with many more nodes equaling more foliage, or the blue spectrum shorter than mixed and with more nodes and foliage. But hey, we are going to find out soon anyways, because im about to buy four 50w all blue LED boards to add to my four HO 6500k Flo's!
I14 have already played with the mixed spectrum boards and i personally don't like, im selling them. If anyone wants them PM me