Is there a discernible difference between tube and pan watercolors?
There can be more textural/quality difference between two different pigments of the same brand and type of watercolor than there is between, say, a tube of ultramarine blue vs. a pan of ultramarine blue from the same manufacturer.

The difference between the types of watercolors really comes down to technique: the way you use tube colors is quite different from the way you use pan colors. But you can use this to your advantage. For example, you can use the pan colors for light washes, and use the more concentrated tube colors for adding outlines and details. To someone who doesn't know what kind of paint you used, it'll just look like you used more water in some parts and less water in other parts (and if you come right down to it, that'll be exactly what you did).

If there is a huge disparity in quality between the types of paint - for example, if the pan colors are a dollar-store find meant for children, while the tubes are top-of-the-line artist's grade - that difference will probably be apparent in the finished product: the lower quality paints will look washed-out and chalky, and may be under-bound (so they come right off the paper when dry). But if both types are student-grade or better, then a competent artist will have no trouble creating a seamless piece of art using both types. It's just that most artists develop a preference for one over the other.