The florets and stems of broccoli are very similar in their nutrient content, and both are excellent parts of broccoli to include as part of your Healthiest Way of Eating. The amount of most B vitamins, minerals, and fiber are very similar in the two parts of the plant (on an ounce-for-ounce basis). The largest difference I've seen documented in the research literature involves beta-carotene, which is about seven times more plentiful in the florets than in the stems. On an ounce-for-ounce basis, the darker green florets also contain more chlorophyll than the lighter green stalks. Since the stalks take a little longer to steam than the florets, it is best to begin cooking them first and after about two minutes add the florets to the steamer.
You didn't mention broccoli leaves in your question, but the leaves of the broccoli plant are also excellent sources of nutrients. They are actually higher in beta-carotene than the florets and can contain phytonutrients that aren't found in the stems and florets.
For more information on this topic, please see:
Shelp, B. J. 1987. Plant characteristics and nutrient composition and mobility of broccoli (Brassica oleracea var. italica) supplied with NH+4, NO3 or NH4NO3. Journal of Experimental Botany 38: 1603—1618.
Food Processor Version 7.71, Database Version December 2001. ESHA Research, Salem, Oregon.