It depends on a) why you're upgrading the old system, & b) how long you plan on keeping this new system.
If you're replacing the old system because the old system just can't be upgraded anymore (i.e. uses a socket even older than LGA 1150, uses DDR2 RAM, has an AGP video card slot, etc.), but your plans for future game/app usage are modest, then a Haswell build will be more than sufficient. We're nowhere near being close to the Haswells being the "minimum", & even if they're starting to be replaced on the "recommended" spectrum by Skylake, as @rolli59 pointed out the current performance bump from Haswell to Skylake is extremely negligible.
On the other hand... if you're wanting to have access to DDR4 for your upgrades, and are that worried about being able to upgrade just about everything in your new system over the next couple of years, then a Skylake build will probably work better for you. You may have to be a little more conservative in the hardware you buy now, though.