I would assume it's a matter of build quality. If the Napoleon is built as well as the Weber, then it would likely come down to which grill has the features you like most. Personally, I wouldn't want a porcelain coated cast-iron grate or a hinged lid. However, the thermometer placement on the Napoleon looks much better than Weber's placement and the included heat deflector is a really nice feature.
I would say your best bet is to go see both in person and get a feel for them.
From the reviews I've seen online, airflow may be an issue with the Napoleon — you definately need good airflow to effectively grill. I've never used a Napoleon, but both of my portable Weber grills have better airflow than any other charcoal grill I've ever used.