Which 501 tubeset? Which 531 tubeset?
Look here:
http://www.equusbicycle.com/bike/rey...-transfers.jpg
Generally, the answer is weight, but there are many factors involved. Are all of the tubes 501 or 531, only the main three tubes, or is there a mix of tubes? Is it plain gauge or butted?
Here's a chart that compares the gauge and weight of a variety of common tubesets (I assume, but am not certain, the overall weight refers to frames with a 'full' tubeset of each variety - main tubes, stays and forks):
http://www.desperadocycles.com/The_L...per_Tubing.htm