for the kind of use your Mom is going to want it for, I doubt there will be much difference between providers. Unless she's some black hat or wants to play Ever-crack for 48 straight hours or needs milliseconds quickness to frag some kid in UT2004.
The easiest way to go about it is sign up for whatever her local phone company offers. It'll come in on the same phone line, show up on the same phone bill, it just saves a ton of time and aggrevation whe it comes to support.
My Mom uses SBC for her phone service, and I got her signed up for Yahoo by SBC. It took a week from order date until the tech showed up(but to give them credit, they set an exact appointment date and time and were right on the dot for both). He ran the line from the outside box to the house, set up her DSL modem for her, (side note, SBC's DSL modem's have built in 11G wireless already), so he set up her WEP key and then showed her how to set up the wireless on her laptop. He went way above and beyond in my opinion. So my Mom told all her neighbors and they signed up too, and got the exact same service, from a variety of different techs. One installler even used his own laptop to pull down the right driver for the customers NIC card so it's get set up.