IMO MLB is a better sport to bet than football. Why? First off, there's many more opportunities. What is there, 275 regular season NFL games? That's not that much. Secondly, there's more useable data. Third, the new frontier is the prop market. There's so many great props in MLB (granted with low limits). I can make 8 prop bets in a day and can easily explain and justify them. When it comes to NFL props, I find it much more of a coin flip. I also think with so few games, EVERYONE knows everything about every NFL team. It's not just professional bettors and books. Nowadays even squares and weekend warriors can be totally up to date on every team.
Don't get me wrong, I love the NFL. Betting it and watching it. And while I will watch MLB, the 3 outcomes obsession has clearly hurt the watchability and made the games longer. But my MLB betting handle will be exponentially higher than my NFL handle.