skylake will be faster, especially when overclocked. the dell does have a small advantage with adding a second cpu, but dual xeon loses alot of performance in overhead. adobe's software has lots of problems with multi-threading, so a dual xeon would take another performance hit there, mostly in photoshop and lightroom. i would recommend looking at a haswell-e x99 build, like the i7-5820k, to fit in your budget. when overclocked it will be somewhat close to the dual xeon performance. skylake and haswell-e support faster ddr4 2133mhz memory than the xeon, and most of their motherboards support non-spec/overclocked memory like 2400mhz and faster. most z170 and x99 motherboards also have an ultra/x4 m.2 slot, so you can use a samsung 950 pro instead of intel 750. xeon's ecc memory usually runs more expensive, so the other i7 builds may help save money there.
there are lots of x99 build threads here if you want to look thru for build ideas too.
Thanks those are helpful thoughts and helped me shy away from the Xeons. I do think I can do a DIY build with a z170-- most of the motherboards I am looking at have two m.2 slots and one has 3, although it seems that the bandwidth may be shared, I am still reading about that. I am seeing the Intel 750 as lower cost right now than the Samsung 950 pros (.70/GB vs. .68/GB), Would there be a reason besides cost to go with the m2 slot over the PCI interface of the Intel 750?
samsung has slightly better stats on their ssd's, but the intel 750 is still good if you want to go that direction.
i really like the idea of the intel 750 with it using a pcie slot and having a big heatsink, and i think that's where desktop computer storage will go. but samsung only offers prosumer/consumer versions of these ssd's as m.2.
the skylake cpu prices have been rising and getting overpriced. if you don't want to do a $2k i7-5820k build, you may want to look at an i7-4790k build as they have been going on sale.