Well, a better cooler will keep food hot or cold longer than a less insulated one. Whether that makes a difference depends on how long you plan to hold. But for reference, a cheap Coleman cooler will keep a large brisket or butts hot enough to serve for at least 5 hours.
One thing I'd mention is if your brisket is done, don't FTC until you let it cool off a bit, especially if you are holding for a long time as it can 'overcook' in there.