Hi,
I have worked with product based firms as well as services based ones. The first thing I have experienced is that the quality of work cannot be graded just on this differentiation alone. I have worked with a few age old products for the print industry on
Java platforms in which I have faced even database design issues. But the honest truth is no one wants to modify already existing code and they would rather develop a work-around solution to fix any bugs that may arise due to such basic flaws. Another problem is unless the product is a state of the art, latest technology based one there is more chance of having to work on older technologies than the latest hot ones. Service industry I feel will require constant up gradation of technical skills.
Coming to on site opportunities what the other poster had to say is correct. And to add it also depends on your position. A PM or a PL stands a far better chance for on site compared to a developer. In services it depends mostly on luck.
Cheers,
Raj.