Since your machine is qualified to upgrade all the way to OS 10.11, El Capitan, it would be best if you upgrade to a higher OS. One reason is that as time goes on, more and more software will be become less and less compatible with older versions of the Mac OS. As it is, you are basically 4 versions behind, given that El Capitan just came out.
The first thing to check is how current are you with all the software you use, especially any non-Apple stuff. If you have been keeping up to date, and given that Yosemite, OS 10.10.5, is still a solid OS (OS 10.11 has some issues for some third party software, so far), it would be wise for you to upgrade to OS 10.10.5. Then, when you are comfortable with it, and OS 10.11.x becomes more "stable", you can then upgrade to OS 10.11.x.
The other thing that is critical is to make sure you are making backups. It would be better if you use a program like SuperDuper! (there is a free version), as it makes an exact backup/clone of your current system, and if you have an issue with upgrading, you can easily recover from that backup. So, let us know what type of backup you are doing, and we can help you further with upgrading the OS.