Volkswagen or Yugo?
each will transport you from point A to B, but there are more choices both above and below....well, not below a Yugo and that's the point :-)
The quality of a VoIP connection is a function of bandwidth to a point. Faster connections will give you better service, So will better engineering. In our house, we have Ooma, 8x8, Magic Jack, and the cable company's service, along with a land line. All the VoIP alternatives suck when I get on the phone and start using the computer and I have a 16 down line. The land line has a buzz and the cable company line seems relatively good through all of this. Cost? Cable line is about $35, so is the Internet service, Ooma cost about $250 for "unlimited" service, 8x8 is about $30, and I scored 6 years of MJ for about $90. My landline is about $60 with special features. I don't discuss my cell phone plan....it depresses me.
I guess it comes down to what do you need a land line a cell phone and the internet to do? Where do you live and how do people call you? Do you need to be in the directory? We get listed in the phone book via the cable company phone and the land line. If you port that number to a VoIP line, you may have to pay extra at directory time. Ooma ports, Vonage ports, Sometimes it seems that our land line is just a place for the telemarketers and bill collectors to call. Our friends have the cell phone number and we're all "IN" so it's free. My internet connection is so fast, I get spoiled when I have to use slow connections outside of my home office. If it were my choice, I'd grab a faster inernet connection and skimp on the telephone service. The cell phone is negotiable. Of course, you can hint to AT&T about your unhappiness and they may pony up a better offer. Get it in writing as they can't violate state tariffs on phone rates, they're regulated.
Another offer on VoIP to consider is this http://www.phonepower.com/services/voip/resdetail.aspx?pid=71
Have fun searching.
Carl