I like them both but for different reasons. Taco Bell is honestly a higher class of food than Jack in the Box. (not by much though) Their food is clean and for the most part doesn't have as much oil content. At least not as much that you notice. Unfortunately they tend to get rid of their best food items, i.e. white corn tacos, big beef tacos, mexinuggets.... I will say that their Chicken Caesar Stuffed Burrito is really damn tasty however. (Though misleading; it's just a Caesar salad wrapped in a tortilla. To be fair though, everything tastes better in a tortilla. Case in point: Double Decker Tacos)
Jack in the Box is great for quick, reliable food. I have never gotten anything unexpected at any Jack in the Box, ever. Even that creepy one down near the Mexican border. True, all the ingredients in their tacos taste the same, be it lettuce, cheese or meat, but it tastes pretty good with some hot sauce. Their Teriyaki Chicken Bowls are pretty good if you can get them to get rid of those pesky vegetables and give you extra sauce. And as for the Bacon n' Cheddar Potato Wedges... mmmmmm. Sure, I know it isn't really cheese, probably doesn't even contain lactose, but it tastes good.
And on the subject of Jack in the Box I really have to question their business sense. Sure, they're doing pretty good, but because they stay open so late (usually later than any other joint in town) they effectively have a business model based on serving the needs of hungry potheads. If you go to a Jack in the Crack after about midnight there will guaranteed be someone a number of cars in front of you going, "dude, do you have steak? I could really dig some steak, or-- ooooo! Nachos! Dude, do you have nachos? Or tacos! Wait... dude, what if a taco... and a nacho... had a kid? It'd be a naco! Dude, I hella want a naco!" And do they have either steak or nacos? No. This should change. You're inconveniencing your hungriest consumer base Jack! Of course they don't remember you inconveniencing them most of the time...