I think it depends which 105 and ultegra cranksets you're talking about - I would prefer the new 105's FC-5600 hollowtech crankset with external BB over an Ultegra 6500 crankset and octalink bottom bracket.
For upgrading old bikes, or building new, last month I saw JensonUSA (or maybe it was Nashbar) clearing out Ultegra 6500 cranks for $49.95. They also had 105 octalink BBs for $22.