As a diabetic, I do not think it would matter. granola bars, and muffins are similar( grains), and if they are in the same range, would have about the same effect on your blood sugar. The only thing that might make a difference is the fiber content. The bar probably is higher than the muffin, unless the muffin is very coarse.
Since you don't have diabetes though, one muffin will not do much at all. If we got off track from one muffin, or a few hundred calories, out of the thousands we eat each week, we would all fail. This is your diet, not a science experiment, you need to ease up, and just eat the muffin.. 14 muffins, 2 a day for a week might be an issue, but one muffin isn't the problem.
Make sure the muffin IS close in macronutrient components, and calories though.. Many muffins are 400 calories, and full of sugar.