Greetings from Nebraska - I started with Slimline but quickly transitioned to a Comfort style by buying Comfort Center Bands and Bushings to convert my Slimlines into Cofmort - without the rubber grip. Essentially a Slimline with a larger diameter center band. The benefit was that I could continue to use the less expensive Slimline kit and still turn a heavier pen without so much corset like appearance because of the tiny Slimline center band.
In keeping with the 7mm kits, I next bought a collection of PSI Designer NT, Euro style kits, then Cigar.
The next was a step up to the Jr. Series to try using Rollerball instead of the Cross and Parker style refills. I use the basic Junior Gentleman kits from "The Woodturning Store" as they are in the same price range as Cigar kits.
Nearly all of my finished pens are made up of these few styles with the exception of a few Sierra and special themed styles.
Good Luck and Best Regards,
Dave
PS Most of my pens are either given away to friends, made for our Church to give as gifts to graduating seniors, or are just collecting in my private collection. The Church pens were a Sierra style called "Elegant Monarch" with little crosses on the center band. The kits were a little more expensive, but I could get 2 pens from a single "Bethlehem Olive Wood" blank since they are a single tube kit.