i3 6th gen or i5 4 th gen

It's a really close call. I took a look at their listed capabilities. Aside from some of the more esoteric extended instruction sets, it seems like these are the main differences:
-- 3 of the 5 mobile Skylake chips have "configurable TDP", where the chip can apparently be programmed to provide specific & guaranteed performance depending on the heat levels/cooling available (i.e. rather than it cycling up until it gets too hot then throttling down until it's cooled off, it can be set to provide a constant level of performance).
-- Heat-wise, Haswell actually has the TDP winner, as the Haswell-ULX chips (i.e. i5-4300Y) have a TDP of only 11.5W. The Skylake-U chips match up with their Haswell-ULT compatriots (15W for low-power models, 28W for standard models), & the margin the Skylake-H chip has over the Haswell-MB chips is very slight (35W vs. 37W).
-- While both the Haswell i5 & Skylake i3 chips are 2C/4T CPUs (dual-core with HyperThreading), only the i5 chips have Turbo Boost. While about half of the Haswell i5 chips have base speeds lower than the Skylake i3 chips, it looks like all of the Haswells can surpass their direct Skylake counterparts on the 2-core Turbo setting. Example 1: i5-4360U (best low-power Haswell-ULT) has 1.5GHz base, but can boost to 2.7GHz with Turbo (or 3.0GHz if only using 1 core), while the i3-6100U (best low-power Skylake-U) is limited to 2.3GHz. Example 2: i5-4308U (best standard Haswell-ULT) has a base of 2.8GHz, & can boost to 3.1GHz with both cores or 3.3GHz on a single core, while the i3-6167U (best standard Skylake-U) is limited to 2.7GHz. Example 3: i5-4310M (mid-range Haswell-MB, 37W TDP) has a base of 2.7GHz, & can boost to 3.3GHz on both cores or 3.4GHz on a single core, while the i3-6100H (sole Skylake-H chip, 35W TDP) is limited to its 2.7GHz base. I know that Skylake is supposed to have a mild performance improvement over Haswell, but that's what, maybe 5-15%? That's roughly the offset that Turbo Boost gives the Haswell core i5 chips.
-- Graphics is a bit of a toss-up. Since we're talking about 2C/4T laptops, they're going to need a) a lot of RAM to use the integrated graphics, or b) a separate GPU for switchable graphics. Suffice it to say, though, that both the Haswell i5s & Skylake i3s are a tossup, with the lowest using Intel HD 4400 & the "best" using Iris 530 graphics, but they're apparently evenly ranked on Intel's "GPU Tier" scale ("GT1" < "GT2" < "GT3" < "GT3e", & both the Haswell & Skylake chips split about evenly between GT2- and GT3-rated integrated graphics).

In short....it's going to depend on a) which particular Haswell i5 & Skylake i3 chips you're looking at, b) how much RAM the laptop has (both stock & maximized), 3) whether the Skylake system is using DDR3L or DDR4 RAM, & 4) which laptop has an SSD.