IMHO, the "best" pen mandrels are the adjustable ones. You can make those exactly the length you need, thus minimizing "mandrel whip" which contributes to out of concentric pen barrels.
Additionally, when you "out grow" the mandrel configuration, replacement "rods" are available cheap ( $6-$7) so that you can continue to do closed end pens without buying or making a load of other "crap".
Also, most rookies tend to overtighten fixed-length mandrels,
causing a bowed mandrel. It is very difficult to overtighten and bend an adjustable.
Lastly, the last time I checked the priTce difference between a fixed length and adjustable mandrel was less than the price of a replacement mandrel shaft.