i3 6100 vs i5 2400 for gaming only

For games that strongly depend on single core performance, the 6100 is the clear winner.

http://cpu.userbenchmark.com/Compare/Intel-Core-i5-2400-vs-Intel-Core-i3-6100/803vs3511

Also, that is a fantastic deal, even for a used 6100.

You won't need an aftermarket cooler for either one, as the stock coolers will be more than sufficient.

As to which one is better...it really depends on the games you're going to play. In many games, they're pretty much neck-and-neck (although testing was done with an i5-2500K, the performance is going to be pretty similar to the i5-2400): DOOM (http://www.techspot.com/review/1173-doom-benchmarks/page5.html), Titanfall 2 (http://www.techspot.com/review/1271-titanfall-2-pc-benchmarks/page3.html), Tom Clancy's The Division (http://www.techspot.com/review/1148-tom-clancys-the-division-benchmarks/page5.html), Fallout 4 (http://www.techspot.com/review/1089-fallout-4-benchmarks/page5.html), & Gears of War 4 (http://www.techspot.com/review/1263-gears-of-war-4-benchmarks/page4.html) all show them neck-and-neck. In some cases, though, even the i3-6100 is beaten by a Sandy Bridge i5, like in Overwatch (http://www.techspot.com/review/1180-overwatch-benchmarks/page5.html) or Star Wars Battlefront (http://www.techspot.com/review/1096-star-wars-battlefront-benchmarks/page3.html), where it beats the i3-6100 by ~21%. So far, the only one I found where the i3-6100 would beat the i5-2400 (only because it beat the i5-2500K) was Rise of the Tomb Raider (http://www.techspot.com/review/1128-rise-of-the-tomb-raider-benchmarks/page5.html), & that was by ~11% (or about the difference in their core clock speeds, 3.7GHz vs. 3.3GHz).

Those are all games that have been released in the last 12-13 months. Going back further is hard, because not many places will go back & re-benchmark with a newer CPU. But, given the increase in core clock speeds from the i3-4130 (3.4GHz) to the i3-6100 (3.7GHz), & based on the benchmarks they show for the i3-4130 vs. the i5-2500K for Project CARS (http://www.techspot.com/review/1000-project-cars-benchmarks/page5.html), The Witcher 3 (http://www.techspot.com/review/1006-the-witcher-3-benchmarks/page5.html), & GTA V (http://www.techspot.com/review/991-gta-5-pc-benchmarks/page6.html), it's going to be another situation where the i5-2400 & i3-6100 are going to be running right alongside each other.

It really doesn't surprise me, though. Skylake only offered an incremental performance improvement (in terms of IPC) over Haswell, which itself only offered a slight performance over Ivy Bridge... &, from what I can tell in those benchmarks, Ivy Bridge was also only incrementally better than Sandy Bridge. And even with the higher clock speed & IPC improvements, the i3-6100 is still just a dual-core that can pretend to be a quad-core by cutting the resources in half...as shown by the multi-threading synthetic benchmarks (http://www.anandtech.com/bench/product/1682?vs=363), which show the i3-6100 losing every single multi-threaded benchmark to the slower i5-2400.

So...

Since you said you're not worried about future upgradeability, it comes down to 2 main questions:

1. What features do you want on your motherboard? If you want DDR4, M.2 SSD, & USB-C, you're pretty much forced to go with the i3-6100 (& even then, that last choice might be very limiting). If, however, you're more than happy with a plain SATA III SSD, DDR3 RAM, & USB 2.0/3.0, then you can still find LGA 1155 boards that will provide what you're looking for.

2. What budget do you have? Basically, which build is going to be cheaper while giving you the features you want. The i5-2400 looks like it's running about $50US more than the i3-6100...but if you can save $50US on the other components, then it comes out as a wash in the end.

IPC is not a fixed thing. In some cases, Skylake is much faster per clock than Sandy Bridge:




In others, the improvement is next to nothing:




A Skylake i3 has slightly less multithreaded performance than a Sandy Bridge i5, and a lot more single-threaded performance.

The reason I favor the i3 is not raw performance though, bur rather the platform its on. DDR3 production is dropping and DDR4 is coming up, so expect future RAM upgrades to be cheaper if you have the i3. The i3's TDP (basically power consumption) is around half that of an i5 2400, so it's a much cooler and quieter running CPU, and can be run with a smaller/cheaper power supply. The i3 is on Intel's latest socket, which will be receiving at least 2 more generations of CPUs that fit into it, giving an excellent upgrade path - though granted, you can still find an old i7 to replace the i5 with. Socket 1151 boards also have things 1155 boards will not, such as M.2, DDR4, USB 3.1, SATA Express, newer/better onboard audio codecs, and display port to name some of the more important ones. Probably most important of all though is that the i3 system will have a warranty on all of its parts, whereas the i5 will not.

It may become tricky (more expensive) in the future to maintain (repair, replace motherboard) for the i5 2400.

The way you put your question, definitely i3 6100.

In your case, I would also:
- do research on costs of intel 4xxx i3/i5 series system in your area / AMD Athlon X4 860K based build (may offer better price/performance)
- see if you can hold off on the purchase for AMD zen cpus to be released and quite possibly lower cpu pricing overall.

IPC is not a fixed thing. In some cases, Skylake is much faster per clock than Sandy Bridge:




In others, the improvement is next to nothing:




A Skylake i3 has slightly less multithreaded performance than a Sandy Bridge i5, and a lot more single-threaded performance.

The reason I favor the i3 is not raw performance though, bur rather the platform its on. DDR3 production is dropping and DDR4 is coming up, so expect future RAM upgrades to be cheaper if you have the i3. The i3's TDP (basically power consumption) is around half that of an i5 2400, so it's a much cooler and quieter running CPU, and can be run with a smaller/cheaper power supply. The i3 is on Intel's latest socket, which will be receiving at least 2 more generations of CPUs that fit into it, giving an excellent upgrade path - though granted, you can still find an old i7 to replace the i5 with. Socket 1151 boards also have things 1155 boards will not, such as M.2, DDR4, USB 3.1, SATA Express, newer/better onboard audio codecs, and display port to name some of the more important ones. Probably most important of all though is that the i3 system will have a warranty on all of its parts, whereas the i5 will not.