Ok. Two things to remember,
1) It is not only her prom, but yours too.
2) Most high school girls love attention.
Ok, now. Everyone wants to remember their prom, but most just tell the same story of a limo or riding with friends. Wouldn't you want to tell your buddies down the road that you went to prom in a Roll Royce or 81 DMC-12? I know i drove my 69 camaro to prom, and I'll always remember it.
Now, if she does like attention, then pulling up in a limo would be like following everyone else. But to pull up in one of those would not only turn heads, but guarantee people to talk to focus on your date.
I might be wrong, but thats my opinion. Go for the Roll Royce.
