Background Research
To start my research, I looked up opinion polls for each subject on www.google.com. I first used the terms "Public Opinion, Legalizing Marijuana", and I found a few results. The first was from an organization dedicated to legalizing marijuana, NORML, who reported that 8 out 10 people surveyed felt that marijuana smokers should not be treated as criminals. The wording used suggests a possible bias, as the question might have asked whether people surveyed preferred marijuana users to get jail time or rehab. I can't confirm my suspicions, but they had a more helpful figure. They claimed that 40% of respondents favored legalizing small amounts of marijuana.
Other Google search results lead to polls regarding medical marijuana use, and general arguments for legalizing or not legalizing marijuana. I changed terms to "Poll, Legalizing Marijuana", and found various polls on legalizing marijuana for medical purposes, but I was more concerned with recreational use. I added "-medical, recreational" to the search, and continued. I found many results on election initiatives to make possession of small amounts of marijuana a misdemeanor. I didn't find any survey, poll or such study done nationally on legalizing marijuana completely. So I changed "poll" to "survey", but got similar results as before.
I changed my search terms to "Legalizing recreational marijuana, -medical, survey", and I found one particularly useful-looking site called "Ya-hooka". The Google result emboldened the words "Marijuana Survey", which turned out to be a link to another site from "Ya-hooka". Alas, the Survey site lead to a generic search page, indicating that any site that was there previously was now gone. Frustrated, I resorted to Wikipedia. "Legalizing Marijuana" led me to an internationally-oriented page. The article linked to a US-specific article: "Decriminalization of cannabis in the United States." Once again, I failed to find a survey addressing my concerns.
I decided to move on to the other half of my project: prostitution. I found many informal polls of the users of various websites. The first four results showed strong support for legalizing prostitution (with or without government regulation). The only formal poll was conducted by the Las Vegas Review-Journal. The article described a relatively well-designed survey of Las Vegas residents that showed that 60% of residents did not favor legalizing Marijuana, with 35% favoring it. This was a shock, because I figured that if any city would want to legalize prostitution, it would be Sin City.
The most credible surveys found on each issue reported similar percentages of support for each issue, with legalizing Marijuana. Each study is flawed, though. The poll conducted by NORML isn't described in-depth, and it cannot be assumed that an organization that's stated objective is to legalize pot ran a completely unbiased study. The poll done in Las Vegas does not encompass the entire U.S.