The dial up connection has a speed of 56 kbps normaly but a broadband connection has a speed of even 10 mbps or more.See the difference...For better voice clarity we need a better downloading and uploading speed.So always broadband is better than dial up connection for VOIP.
Now a days no one is going for any dial up connection. Everybody needs only broadband since it is 100 times faster than dial up connection.It is giving a faster uploading and downloading facility.
It takes approximately 35 mins to dow load a 5 MB in a dial up connection song and when using broadband it takes just 35-40 seconds to download it. You can also make phone calls using voip phone system anywhere at very cheaper rates.
Basically broadband is better due to the speed.Also you won't have any problems with your line disconnecting or anything like that it should hopefully all run smoothly.