SIPphone for iphone

hi people,

i am planning to buy an iphone or blackberry, either of the two.. since i have an account for voip, i am going to ask what's best sipphone for it? coz i have a friend who has an HTC windows mobile 6, she tried to use fring to configure her account, but she said fring is not good enough to makes any calls and receive any calls, she gets bad quality of voice for it. so since i have not yet buy an iphone or blackberry, i ask in advice and get some advice to those people who are expert on this matter so that i will not encounter such problem like my friend had when i got one mobile.
thank you!

wendyL