Based on personal experience and some recent posts, it seems that a ripped CV boot is purely a case of bad luck, rather than poor manufacturing or maintenance. The recommendation to replace the shaft sounds like it must be more cautionary (and/or greedy) than absolutely necessary, since you can't tell if there's a problem with the CV joint unless it's making a noise (or you pull it apart and inspect it). So, unless it is,in fact, making noise, I would go with the second opinion (replace boot, repack joint with grease). The procedure would require the mechanic (if he's a good one) to take the joint apart and clean it, so if he notices a problem while he has the joint apart, then you can decide to replace it.