In terms of overall gaming performance, the Intel Core i5-3427U 1.8GHz is very slightly better than the Intel Core i3-3120M 2.5GHz when it comes to running the latest games. This also means it will be less likely to bottleneck more powerful GPUs, allowing them to achieve more of their gaming performance potential.
The Core i3-3120M 2.5GHz was released less than a year after the Core i5-3427U 1.8GHz, and so they are likely to have similar levels of support, and similarly optimized performance when running the latest games.
The Core i3-3120M 2.5GHz and the Core i5-3427U 1.8GHz both have 2 cores, and so are quite likely to struggle with the latest games, or at least bottleneck high-end graphics cards when running them. With a decent accompanying GPU, the Core i3-3120M 2.5GHz and the Core i5-3427U 1.8GHz may still be able to run slightly older games fairly effectively.
More important for gaming than the number of cores and threads is the clock rate. Problematically, unless the two CPUs are from the same family, this can only serve as a general guide and nothing like an exact comparison, because the clock cycles per instruction (CPI) will vary so much.
The Core i3-3120M 2.5GHz and the Core i5-3427U 1.8GHz are from the same family of CPUs, and thus their clock speeds are directly comparable. With this in mind, it is safe to say that with a 0.7 GHz faster base clock rate, the Core i3-3120M 2.5GHz manages to provide significantly better performance than the Core i5-3427U 1.8GHz.
Aside from the clock rate, the next-most important CPU features for PC game performance are L2 and L3 cache size. Faster than RAM, the more cache available, the more data that can be stored for lightning-fast retrieval. L1 Cache is not usually an issue anymore for gaming, with most high-end CPUs eking out about the same L1 performance, and L2 is more important than L3 - but L3 is still important if you want to reach the highest levels of performance. Bear in mind that although it is better to have a larger cache, the larger it is, the higher the latency, so a balance has to be struck.
The Core i3-3120M 2.5GHz and the Core i5-3427U 1.8GHz have the same L2 cache size, and the same L3 cache size, so in terms of cache-related gaming performance, we have to look back to the clock rate, where the Core i3-3120M 2.5GHz wins out.
The maximum Thermal Design Power is the power in Watts that the CPU will consume in the worst case scenario. The lithography is the semiconductor manufacturing technology being used to create the CPU - the smaller this is, the more transistors that can be fit into the CPU, and the closer the connections. For both the lithography and the TDP, it is the lower the better, because a lower number means a lower amount of power is necessary to run the CPU, and consequently a lower amount of heat is produced.
The Core i5-3427U 1.8GHz has a 18 Watt lower Maximum TDP than the Core i3-3120M 2.5GHz (though they were created with the same size 22 nm manufacturing technology). What this means is the Core i5-3427U 1.8GHz will consume slightly less power and consequently produce less heat, enabling more prolonged computational tasks with fewer adverse effects. This will lower your yearly electricity bill slightly, as well as prevent you from having to invest in extra cooling mechanisms (unless you overclock).
The Core i3-3120M 2.5GHz and the Core i5-3427U 1.8GHz both have an on-board GPU, which means that they will be capable of running basic graphics applications (i.e., games) without the need for a dedicated graphics card.
For an in-depth GPU comparison, click on the GPU comparison icon that you can find throughout Game-Debate:
On-board GPUs tend to be fairly awful in comparison to dedicated cards from the likes of AMD or Nvidia, but as they are built into the CPU, they also tend to be cheaper and require far less power to run (this makes them a good choice for laptops). We would recommend a dedicated card for running the latest games, but integrated GPUs are improving all the time and casual gamers may find less recent games perform perfectly acceptably.