I'd prefer to see a spread, however, many people may not know what +13 and -13 means.
To solve this problem, in the comments below the team logo, you could put -13 (Alabama must win by more than 13 points.)
If using a spread, you'd have to determine what will happen should Alabama win by exactly 13 points.
- The earnings could carry over to the next game.
- The prize money could be split between everyone who voted
- Rather than only having Alabama and Tennessee options, you could have a 3rd option for a "Spread-Adjusted Tie (Alabama wins by exactly 13 points)"