My understanding is that in today's market, fixed-odds betting is all about making sure betting either side is -EV, and spreads are all about keeping equal action on each side.

For a quick example of what I mean about fixed-odds betting, Bovada has a baseball game for today at Rays +113 Yankees -133. You need the Rays to win at least 46.95 percent of the time (or more) to turn a profit, and you need the Yankees to win 57.08 percent of the time (aka Rays win less than 42.92 percent of the time) to turn a profit. This means they believe the Rays will win between 42.92 percent of the time and 46.95 percent of the time. If they're correct about this, then neither bet can turn a profit.