I don't know that I'd recommend buying a more recent motherboard/chipset as an upgrade, but saving $75 or so by re-using DDR3-1600 RAM.
For one thing, there are comparatively few Z170 boards that will run DDR3L, so your choices would be limited (which in turn will limit price options). But more significantly, the speed: Here is a comment by a Corsair rep over on their website, from back when the question of using DDR3 on Z170 was current:
"DDR3L offerings for Skylake stop at 2133MHz, while DDR4’s clock ceiling just keeps rising."
>LINK< Now, a lot of people are going to jump all over the speed question, saying that in order to get faster speeds than DDR3, DDR4 has higher latency (lower latency is better). Well, here's what Corsair has to say:
"The real question and concern most users have when it comes to DDR4 is the higher latency, but as it turns out, this isn’t a very significant issue. DDR3-1600 has higher latency than any DDR4 on the market, while DDR3-2133’s latency is only marginally lower than DDR4-2400"
I believe it accurate to say most Z170 chipsets will support DDR4 that easily runs twice as fast as DDR3-1600 (and beyond). So again, I don't know why you'd want to spend what it will cost to replace a motherboard and CPU worthy of an upgrade, then re-use memory that's half the speed just to save ~$75.
Also, a technical point: DDR3 and DDR3L are not the same thing. The "L" at the end stands for "Low power", which means the voltage supported is on the order of 1.35v instead of 1.5v. It looks like the Z170 boards that use DDR3 actually support DDR3L, and not all DDR3 RAM will function properly at the lower voltage level (if it runs at all). So any given set of DDR3 RAM may not even be supported on a Z170 board. I'd say you'd have to be very careful here.
As regards the comparative cost of 4790k vs 6700k: The real difference is going to be in the motherboard chipset. A 6700k basically means Z170 (or Z270), where a 4790k needs a Z87 or Z97 motherboard. As always, the features of these respective boards is what drives cost, and what may be the biggest feature difference is M.2/NVMe support. This brings huge performance gains in terms of storage speed. Z87 doesn't support this, therefore you should be able to get a much better price on a Z87 setup than any of the other three. I'd expect to pay more for Z97, Z170, or Z270 because of this one factor. (It's worth noting that many boards prior to NVMe support like Z87 or even much older can still be fitted with much faster storage than a SATA SSD - but that's a different subject).
I'm also pretty sure that a 6700k setup will outperform a 4790k...in fact, I have examples of both sitting not two feet from me. The 4790k is overclocked to 4600 at the moment, but barely squeaks past the 6700k, which isn't even overclocked at all. There is absolutely no doubt in my mind that the 6700k will pull ahead by quite a margin once I ever get around to properly overclocking it. According to siliconlottery.com, 68% of the 6700ks will hit at least 4700, and that means a 17.5% increase over stock. The 4790k is already pretty much at his limit, but the 6700k has room to grow yet.
So, there's little way I could recommend anyone pay the same for a 4790k setup as for a 6700k. The 4790k choice would obviously be about budget/cost, where (if budget permits, as I said above) the 6700k would be a step up - and cost more (and I think this is still true even if the 4790k was a Z97 v Z87...but it does become a tighter contest). Sheer performance is a factor, but even if you lay that aside, newer stuff costs more as a rule.
If budget is super tight, and re-using DDR3 memory is considered a factor, I'd say save the extra cost and go with a 4790k/Z87 setup that can use the DDR3 RAM. TBH, though, I have to say that a better idea than re-using the RAM would be to sell the older setup as a whole, and (even if you save by staying within Z87) get a setup that includes faster memory. For example, I have two Z87 boards here ATM; both running DDR3-1866 (the manual for one says it will go DDR3-3000, but I haven't tested that).