My take on it....
If you're just getting started, go with a bigger cooler. If you plan to batch sparge, the 9-gallon Coleman Xtreme works great. I do 5-gallon batches, and even relatively small beers (without a ton of mash mass), the cooler doesn't lose any temp (or, less than a degree) in an hour (assuming you preheat the cooler). It wouldn't be ideal for fly sparging, but it's a great cooler for $30. Spend an extra $10 upfront and leave yourself the option down the line of either really BIG beers, or 10-gallon batches.