im pretty sure that vivaz isnt a smart phone, its selling point is the HD camera but x10 does use the android operating system. if your wanting to upgrade to the iphone i dont think that you will be able to negotiate any better deal then whats already on offer. the prices arent set by the providers but by apple themselves unless you get a reconditioned one. thats where its been sent back within the two week cooling off period, so its considered second hand. bear in mind that some of the reconditioned ones may have just been refused delivery at the door, so they wont have been opened. but because they have been returned, they are slightly cheaper then a brand new iphone.
remember with the iphone you can get unlimited web browsing, and its got 16gb of memory as standard. for other phones your goin to have to purchase external memory if you want to store as much data. not many people have complained about the 3gs, but the main problem with the 3g was the battery life. apparently thats been increased by 40 percent in the 3gs so your not goin to have many qualms with it
hope i'v been helpful, but check this site below, it has alot of information and reviews.
http://www.gsmarena.com/