It seems that when it comes to sales, trailers are better than demos. That's the argument being made by noted Puzzle Clubhouse CEO and analyst Jesse Schell, who spoke at a recent DICE keynote. Using data from consulting company EEDAR's cumulative sales of Xbox 360 titles, Schell was able to conclude that games without demos statistically sell better than those with demos — IF they have a trailer that entices players.
The EEDAR report states that games without a demo or a trailer (there are games without a trailer?!) sold less than 100,000 units on average. The next highest average were for games with just a demo, averaging 200,000 units. Those with both a demo and a trailer averaged 250,000 units; but leading the way were games with only a trailer, coming in at over 500,000.
People see the trailer and they're like 'that's cool!' and they made a plan. 'I gotta try that game!' And then when they play the demo [and they think] 'alright I've tried that game. That was cool. I'm done.' But the things with no demo, you've got to buy it if you want to try it.
Personally, I don't agree with this notion that demos will turn consumers off from buying a game because they've already gotten their fix. To me, if a game is good, regardless of whether a demo is available or not, people will buy it. The report also doesn't take marketing budgets into account. Also, big games will get more coverage and often have Hollywood blockbuster-type trailers to go with them. Smaller games can have a trailer and demo, but they might not get enough advertising to make an impact.
For games that you have never heard of, a demo can only help, not harm them. Think about it: do demos often leave you wanting more of a game or turn you off from it?