They use the same bushings and tubes.. Major difference is how the transmission is installed. With the sierra the transmission screws into the "nib" and your turned body slips over the transmission... The gatsby you will press a transmission "holder" with clip into the the top of the turned body. You then screw the transmission into this hold and a cap goes over the transmission.
I like both of these pen styles very little difference in looks when fully assembled. One thing I like about the gatsby over the sierra: The end user can jam the transmission all the way up in the sierra body when changing the refill. Where the gatsby will not have this problem because it is pressed into the body of the pen.
To prevent this with the sierra, just glue in a "plug" into the cap of the pen.. Just an extra step you do not have to do with the gatsbys.... They are both good kits and great sellers at craft shows.... Best of all, you can turn two pens at a time on a standard mandrel. Both pens are good for bulk orders (IMO).
Hope this helps!
Rick