that all depends on what you want to do for the pictures.. if you want nice upclose portrait like shots, then the 50mm.. the 28-105 is a pretty good all around lens with room to move.. is there a reason yo cant bring them all with you and adjust accordingly??? thats what i tend to do (but I have 2 cameras)
the most generic will likely be the 28-105... but dont be afraid to switch em around, is this your first portrait sitting or no? btw, i'm digging your website.. cool layout.. what did you make it with