It is a common misconception that Shincha is "First flush" and sencha comes from later flushes. I am far a fluent Japanese speaker, so I will not even dare to quote direct translations, and what they mean. Shincha most certainly is unique, some view it as a celebration of the new harvest, and others view it as a bit of a gimmick. In that sense it is quite like the Beaujolais Nouveau of the Japanese tea world.
But Shincha and Sencha both come from the first pickings of tea in Japan. Though both are processed differently, which lead to their different tastes. I personally am a bit more fond of Sencha, as Shincha in my opinion has always been a bit of a kick in the mouth. I liken Sencha to eating lightly cooked vegetables, and raw baby carrots, sweet with lots of other nice flavors. Shincha to me is like eating raw broccoli (without any sort of dip). Some people enjoy it, for others its a bitter mess that must be enjoyed in moderation.
For me to reduce the strong flavor profile of Shincha to one I find more enjoyable I often brew cooler than I would for Sencha, and much shorter especially after the first infusion. Its something like this that makes me like tea so much, with most other beverages their taste profiles can not be customized to taste without actually putting in some sort of other ingredient, with tea you can just brew it slightly differently, to tune it to your tastes.