The problem is that not all providers are available in all areas. The DSL provider must have service in your area. I'd say check the phone book for providers, then look online for reviews. Also ask your neighbors.
I have a cable modem at home, which I love. However, at work I do support for both DSL and Cable services. I can tell you that it's part computer, part service. You can have the fastest service in the world, but if you're running Windows 98 on a 300 mhz computer, things are going to be slow. Alternatively, you could be running Vista on a 2.4 ghz dual-core processor, but have only 128k download speed, and it would still be slow.
The big factors for DSL speed is the distance between you and the local switching station, the quality of the phone lines, and the quality of the equipment. Phone line exposure can also be an issue. I can't count how many customers complain that their service degrades when it's raining. Cable service is also the quality of the equipment, quality of the cable lines, and the number of customers on each node. (I maintain that this is also an issue with DSL, but I'm not quite sure how the lines are run and connected.) So if you're one of very few people on your block using cable internet, you won't be fighting over bandwidth. But if every person on your block has it and the provider hasn't allowed for that, your speeds may be slower than usual.
Oh, when you start talking to the providers themselves, don't always believe what they say about speed. I've heard tales of the phone rep promising that the customer will get great speeds, but then the tech will come out and say that there's no way they can get that where they live. So take what the phone rep says with a grain of salt.
Oh, Carol, if your inlaws have phone line filters, they may need new ones. Bad filters can cause extra line noise, which messes up the DSL.