I recently faced a similar dilemma but found that there was a more basic decision required before picking a Z170 board.
The more basic decision revolved around a choice of socket - to stay with LGA1150 (virtually obsoleted by Sky Lake release), to take the dive into LGA1151 (brand new, initially high pricing) or to consider an LGA2011-3 option (stable, likely to be around until 2017?).
I immediately discarded the LGA1150 option as PC hardware product lifecycles are short enough without buying something at virtual end-of-life.
The LGA2011-3 had some initial negatives (largely perception-based) - expensive boards, expensive CPUs, expensive DDR4 RAM and no on-die graphics.
LGA1151 seemed, at first glance, to be the answer - beginning of life-cycle so like to be supported for longer, DDR3 RAM still useable, access to wondrous new technologies like USB 3.1 and x4 PCIe SSDs, M.2 form factor support, etc, ad nauseam.
In doing some digging around re LGA1151 in general - and Z170 versions in particular - I spotted the increase in PCIe lane support (40) and thought "here was an enthusiast platform capability available on a performance platform".
Wrong! While the LGA1151/Z170 offering DOES provide 40 lanes, the early board examples all route the additional lanes vie the DMI 3.0 link (between CPU and PCH) and NOT direct between CPU and Video (GPU). Also, these wondrous new technologies like USB 3.1 and Thunderbolt and PCIe 3.0 SSDs, M.2, etc. deserved some investigation.
I've yet to find any locally-available peripherals capable of USB 3.1 interfacing and, even if such devices existed, what would they be? Storage is the only area that would really benefit by faster data transfer due to the growing sizes of data/media files. Currently - and for quite a while yet - large capacity external storage (which is the primary usage for some GT variant of USB) will continue to use magnetic disk as storage (= HDD - cost per terabyte low) and HDD technology has some distinct limitations in terms read/write rates which fall well within the USB 3.0 capabilities (fastest SATA HDD is probably Seagate Enterprise 6TB at ~210MB/sec sustained and I get ~250MB/sec between SATA SSD and Sandisk Extreme USB 3.0 flash drive). So, decided to discount USB 3.1 as a deciding factor - a solution hunting for a problem?
Similar thinking re PCIe SSD (initial pricing, board implementations prior to LGA1151 were kluged, etc.) My take is that some future platform (after LGA1151/Z170) will finally get the architecture right so that this storage interface begins to work properly (Maybe with storage-to-CPU being handled in a similar fashion to current CPU:Video interface bypassing the PCH via the inclusion of a "Storage Controller Hub" separate from the "Platform Controller Hub").
M.2/NGFF SSDs still worry me in one area - their weak ability to dissipate heat under high I/O loads (lack of heat-sinking causing thermal throttling?) PLUS the tendency of board designers to locate the slots in parts of the board where ventilation/airflow is obstructed both combine to raise the issue potential problems from overheating.
By this stage my initial enthusiasm for the Sky Lake platform was beginning to fade, so took a more serious look at LGA2011-3 and discovered that the perception re cost was just that - a perception (based on always looking at the top-of-range).
The LGA2011-3/X77 combination supports just three CPU options: i7-5820K, i7-5930K and i7-5960K. Of these, two are 6-core/12-thread CPUs and one is an 8-core/16-thread CPU. Similarly, the PICe 3.0 lane configurations also vary with one offering an entry-level 28-lanes and the other offering 40-lanes. The key difference in PCIe lane options versus Sky Lake being the split of lane allocation between CPU : Video and CPU : PCH interfaces, where the X77 in its 40-lane CPU usages will support 16x / 16x / 8x 3-way SLI usage (if supported by board).
However, if one is not looking for a multi-GPU configuration, the LGA2011-3/X77 entry-level single GPU configuration would comprise:
1) LGA2011-3/X77 ATX board
2) i7-5820K 6-core/12-thread / 28-lane (16x/8x/4x)
3) DDR4-2133 RAM (capacity linked to usage but minimum of 4 x 4GB for quad-channel)
4) A GPU of some type (no on-die graphics on i7-5xxx CPUs)
NB: If multi-GPU is essential, switch CPU to i7-5930K for 40-lane version and you get 16x/16x/8x 3-way SPI capability
So, I would suggest you run a parallel investigation and work up two priced configurations - one for Z170 and one for X99 - then compare the real-world capabilities of each .
Then decide...
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