Aerometric. It seems simpler (simple design is good design: my rule in computer systems). Aside from the breather tube, which, as Mike points out, can corrode or otherwise fail, the aerometric's pili-glass sac seems to last forever. At least, I've never heard of one wearing out, and I used to chat with our sorely-missed friend, Tom Mullane. In fact, one of the free benefits of having Tom repair a pen was the phone calI I would get...a treat when I had a private office because we could talk about kids, kids and pens, weird things he had found inside Parker 51s, people in the repair "industry" ("he's a big guy, so you'd never expect that he has such a delicate touch").
I've seen a 51 aero with a punctured sac, but not a worn-out sac.
Therefore, I expect an aero to work right from Ebay. I've been lucky with breather-tubes. Otherwise, the worst I've seen was a great "English" 51, teal medium nib, that had a block of ink dried solid in the sac. Couldn't begin to squeeze it. Must have been filled and left in a drawer fro twenty or thirty years.
The P51 Vac...ah. Beautiful caps, but usually require someone (other than fumbly me) to re-sac them.
*
An extra "data-point" on the raging question of whether some type of Noodlers ink turns an ink sac to goo: a few weeks ago, I asked Tim Girdler to tune a medium-nib P51 Vac. I write almost always with Diamine Sapphire, or Akkerman-equivalent. Noodler's blues don't appeal to my purple-preferring taste. Tim cleaned the gunk and found remains that look like Diamine, as I expected.. So, if Mike is still keeping score, here's a sac that had one of the friendliest inks but still mushed.
Edited by welch, 19 March 2014 - 07:54 PM.