I don't believe there is any difference in memory usage. It's a question of what's best for you in practice.
On the second question, it really depends on the specifics of what you're doing. I could think of reasons why either could be best. You really have to give it a go and test it out.
If you want to get a better ide of what affects performance, you should search tutorials and the manual. There's plenty of information. Understanding things that are known performance issues can help you make better coding decisions.