I am only looking to print line drawings but my question is would I get better results using an Arduino Mega or a Raspberry Pi or are there better ways to do things with the Uno?
I'd have to say chocolate ice cream..
Of course, I'm sure that you will re-read your question, and realize that there is no intelligent answer to it.
How can we tell whether there is a better way of doing something, when we don't know what you are actually doing? Post your code, so we have a clue.
As to whether a microcontroller with more memory would help, only you know (or can determine) how much memory you are actually using.