Overall, performance should be pretty close. The i5-4200u can turbo boost up to 2.6GHz, but has a base clockspeed of 1.6GHz. As long as the CPU does not get too hot it can turbo boost up to 2.6GHz when one core is used or about 2.4GHz when both cores are used. The i3-6100u simply has a base clockspeed of 2.3GHz.
The both cores can operate base clockspeed unless the CPU is simply overheating and needs to downclock itself to prevent damage. Therefore, the difference between the two CPUs when both cores are being used and when the i5-4200u is not running too hot to prevent the CPU from turbo boosting up to 2.4GHz is basically 0.1GHz (100MHz). The improvement in CPU architecture from 4th gen to 6th gen further marginalizes the difference...