I've discovered Jersey Mikes and Port of Subs since my last Jimmy Johns visit and I can say that while the other two are similar to one another, JJ is a little different. My sub, the Gargantuan, is extremely meaty. More so than the others. The bread is also different. It is more dense. They're all delicious but it's nice to have some differentiation.
One place where JJ can get an extra point is to be able to sprinkle more vinegar on my sub without it having to the the oil and vinegar mix. The amount of vinegar I'd want would make my sub an oily mess. So when I really need to have a more tart sub it'll likely be Port of Subs. But for an otherwise awesome meat-filled monster, bring me my Gargantuan!