I have a set of Starrett, SPI, and Mitutoyo digital calipers.
The Starrett is by far, the best.
I measure aircraft parts as an inspector at work and have the opposite feelings about the Starrett and Mitutoyo digital callipers. Now I will qualify that by saying I haven't used any of the newer Starrett callipers (company hasn't bought any more) in the last 10 or so years but I hated the older ones with a passion. Hope the newer models are better. I prefer the Mitutoyo but they are overkill for most pen turning.
Pete
My brother is a certified master machinist and tool and die maker, so I get a lot of his opinions on this stuff, but one thing is for sure.....We are treading on some very touchy subjects here, along the same lines as people's religion, politics, and relative merits of their offspring!
Huge volumes of opinion, fact, myth, deliberate misrepresentation, and outright BS have been written on measurement instrument makers, models and brands. They inspire loyalty in their adherents comparable to that of the proponents of Chevy vs Ford, Aberlour vs Dalwhinnie and Helmans vs Miracle whip! Like anything else of value, there are hand tuned precision instruments capable of remarkable accuracy, and there is machine made crap with lousy QC and manufacturing practices, and often the same company putting their name on both at one time or another. For the most part you get what you pay for, and unless you are really sure of what you need, and how to make sure you are getting it, trying to get something on the cheap will often get you nothing but grief, no matter what name you buy.
There is no doubt that you can pay lots of money for the name on something that is no better than another at half the price if you are not knowledgeable and careful. You can also pick up a bargain on a fine instrument in the used market if you know what you are looking at. But it's a fine line, and even the pro's occasionally get fooled.
For the purposes of turning wooden pens with hand guided tools on a wood lathe, almost any metal dial caliper with a lock screw that is functioning as well as it is intended to will be accurate enough. Even a tool that is nowhere near machining tolerances is capable of measurements more accurate than the tools we use can reproduce.