VPS CPU Cores v. VPS RAM
When choosing a VPS plan you must decide if more CPU cores or more RAM (memory) is more important to you. Both CPU and RAM are important in being able to run smoothly and quickly. You can find equally priced VPS plans from different providers that are largely the same, but one will have multiple CPU Cores and the other will have higher memory. Whether cores or memory is more important depends on what you will use the VPS for.
The best way to answer this question is to run a test. If you are able to run the applications that you plan to use on your VPS before you buy the VPS then you should be able to monitor how much RAM and CPU the application or program is running.
For example, if you plan to host a website on the VPS, setup the website on a local computer and see how much memory and processing power it uses. This will give you a rough idea of how powerful your VPS should be.
If you are not sure how to test your program before buying the VPS then follow these general guidelines.
If you will be doing a lot of calculating or computation, go with a higher amount of CPU cores. The CPU is the central processing unit. It does all the processing (aka thinking) that the computer does.
If you are serving a website, a higher CPU speed will allow the page to load faster. The webserver will use the processing power of the CPU to create the HTTP response and will be able to respond faster. This of course also depends on things like the Internet uplink speed.
You should also see if the application or program(s) you plan on running can take advantage of multiple cores. Some programs are not able to use more than a certain number of CPU cores. Also, be aware that CPU cores are going to be shared between all the VPSs. If you really need a high amount of processing power you might be better off going with a dedicated server rather than a VPS.
If on the other hand you need to store large variables then more memory will serve you better. For example, I have a script that reads a large JSON file and needs to store a large amount of the JSON data into memory at one time. This requires a larger amount of RAM. I actually ended up upgrading my VPS because the low-end VPS I was using could not handle storing this amount of data in memory.
Caching a website is also memory intensive. Reading from a harddrive is a lot slower than reading from RAM. This is why caching is so fast. Caching stores data in teh RAM and serves it directly from the RAM. No need to use the slow harddrive.
Be aware that it is easier to oversell CPU power than RAM. This is especially true if your VPS provider uses KVM visualization. What this means is that it is easier to put too many VPSs on a single server and there isnt enough CPU to go around. Memory is easier to completely split up so that it is not shared.
If you are using a reputable VPS provider then this should not be a concern. The well known VPS providers do not oversell their servers. It's the VPS plans that offer too-good-to-be-true plans that are the ones who oversell their servers.
If you are still unsure if you should get a VPS with more CPU cores/CPU speed or more memory, go with memory. Unless your programs will be doing a lot of calculations, having a higher amount of memory is probably going to serve you better than a higher processing power. For many programs the bottleneck is the slow speed of harddrives. Having more RAM allows you to overcome this bottleneck.