Really depends on what you're going for. Broadwell (the i5) is slightly more efficient, and hyper threading (the i7) is useful for tasks that utilize many cores/thread (not gaming). For gaming, pick the cheapest one. For multitasking, the i7 wins. Also CPU boss is not a very accurate test.
The i7-4510u is a little more powerful than the newer i5-5200u in terms of processing power because of the higher clockspeed (3.1GHz vs 2.7GHz). However, the i5-5200u can process more is bit more instructions per 1Hz then the i7-4510u so the performance difference is actually a little less than the 400MHz difference in clockspeed. The i7 also has 1MB more internal cache which can help a bit in CPU intensive tasks.
When it comes to integrated graphics performance, the newer Intel HD 5500 is on average around 15% - 20% more powerful than the older Intel 4400; performance varies depending on the game.