It's generally better to wait until 10.X.3 or 10.X.4 before updating. Earlier versions aren't fully patched and there are usually some annoyances, including slightly lower performance.
probably not much difference if you have a SSD and a reasonable amount of memory - i have a 2012 15" i7 and did not notice a difference between el cap and sierra
probably not much difference if you have a SSD and a reasonable amount of memory - i have a 2012 15" i7 and did not notice a difference between el cap and sierra