As you might see discussed elsewhere in these forums, bipolar I is not necessarily worse than bipolar II! As a very broad generalization, they vary more in terms of which pole you spend the most time at than severity per se. With BP I, mania is more pronounced, and psychotic symptoms are sometimes involved. With BP II, depression dominates, but it can be harder to identify because hypomania is not as obvious as mania. In either case, it's a tough set of symptoms to handle, but some meds are better at suppressing mania while others are better at alleviating depression - Lamictal is one of the latter, which is part of why it's one of the first choices for BP II.

Those definitions are just the current diagnostic guidelines, however, and they are going to be revised for the next version of the DSM. So then we can all have new labels, and be extra confused/confusing!