There was some confusion about DDR3 memory support for Skylake as Intel exclusively talked about DDR4 (1.2V) and DDR3L (1.35V) support but ASUS cleared things up and said some of its motherboards will let you reuse your old DDR3 memory.
DDR4 is still more expensive than DDR3, especially if you want high-speed and low-latency memory that outperforms DDR3. So by picking a Z170 based motherboard with DDR3 support you will be able to push your DDR4 upgrade a couple of years down the road, depending on how often you upgrade your system.
For those that might consider upgrading to DDR4 though and want a motherboard to support that, there are some options. The most notable one is the Hi-Fi H170Z3 from Biostar which has 2 DDR4 and 2 DDR3 slots. Given that DDR4 supports 16GB DIMMs, it’s possible to put up to 16GB of DDR3 in now and later upgrade to a max of 32GB of high-speed low-latency DDR4 ram. Hopefully, there will be more boards from other firms for dual DDR3/DDR4 support.