I don't know about the Seattle area, but this is how it is here:
Cable is faster right now because there aren't a lot of people on it. I recommend that every who asks me get DSL so that my bandwidth stays high. Cable seems to be more susceptible to bandwidth degradation due to high traffic. Cable costs less here by quite a bit. It is less than half of the total cost of a low dsl connection.
DSL is less susceptible to bandwidth problems, but I was told by a tech that works at one of the local DSL providers that DSL is not truely dedicated bandwidth as everyone thinks. For simplicity, if they have one T1 and 100 customers with 256K DSL accounts, the customers are not going to get the full use of their 256. They will have 256 to their Service provider, but anything beyond that will be slower. I am not sure if all DSL providers are that way, but I wouldn't bet against it.
Either way, it is better than Dial-Up. I would look at Initial cost and monthly cost as the determining factor. You will probably be satisfied with the speed that either will give you.