8GB or 16GB RAM?

16 since you can't do it on your own.

 

16 is nice, but it depends on use. The problem is that these upgrades for longevity are often bad investments. Much of the time the amount spent aligns poorly with the extra time granted. There's a reasonable chance of hard drive failure within 5 years, so if you want to keep it that long, you should be comfortable with prying it open.

 

16GB

forget about opening it up to do it yourself and voiding the warranty

 

Check your page ins and outs on your current Mac.

8GB is plenty unless you are using some highly demanding software. As usual - it all depends on what you are doing with your Mac.