I'm playing with the same decision about Spiro Weichs or Mittels now.
For those of you who moved to mittels from something lighter, how long did it take to adjust to the mittels? I've been playing perpetuals on the Spada, and I'm trying out a Shen as a backup / cramped gig bass. I like the Shen, and I like the sound of the Spiro Mittels it has on it, but between the increased string height on this bass and the higher tension of the Mittels, my left hand is not happy with me after about an hour of playing. Granted it has only been 3 days, but I'm trying to sort it out while I have it on trial.
There's no room to lower the string height on the adjusters, so I'm thinking of putting on some strings with lower tension. Mostly wondering if I'm bailing too soon on the mittels.