Based on what I've read, it does seem the effects can vary quite a bit from person to person. Personally I would probably go with 18mg before 20mg, considering a 2mg increase can be twice as intense. What other experience do you have with psychedelics? And the most important thing I'd take away from the first experience is the timing of the drug (knowing how long it takes to kick in and last), but that's just me. 20mg will possibly last a bit longer than 15mg, but I don't know that for a fact.
Personally I love trance music on drugs, lol. It really helps add a positive vibe to the trips ime. All of the armin van buuren - a state of trance albums are really good. Gareth Emery and Above and Beyond have some good stuff too
http://www.youtube.com/watch?v=BhKxNBmarvI
http://www.youtube.com/watch?v=CG767w-GtC0