It depends honestly on how much it would cost for the copper line versus the VOIP line. Generally copper is 'standard call quality' unless you have really old and ill maintained lines. VOIP services, especailly the cheaper ones, can vary significantly on call quality, some can be as good as local lines, and others can be worse. You are typically going to be paying less for most of the basic VOIP packages compared to a traditional land line however, especailly if you want all of the features such as call waiting, which usaully are an additional charge with landlines.
One other slight issue with VOIP lines specificly is that not all of them are reliable for 911 service, if that is important to you then you need to verify with the VOIP provider that they are correctly setup to handle 911 emergency calls.
