What is the performance of the Mi-Si pickup?
I have MiSi pickups in three of my ukes (two tenors and the baritone) and under-soundboard pickups in most of the rest of them. The MiSi is a very nice little pickup for live performance, and extremely convenient. It has quite a bit of compression and a slight tendency to accentuate any "thudiness" of the uke but this is easily EQ'd out at the board or with an outboard pedal. The "thudiness" is more noticeable on a tenor than a baritone. (Note that all three of my MiSi pickups are the earlier "one size fits all instruments" model - the new ukulele-specific ones might have EQ built into the endpin preamp to reduce the thudiness, I don't know.)
Again, most under saddle pickups have this thudiness - it's really just amplifying part of the actual sound of the uke but it's a part that isn't as noticeable acoustically because the uke body tends not to be very responsive to frequencies in that range. The big difference is that some other uke pickups have preamps that are tweaked to reduce those frequencies - and, as I said, the newer MiSi uke models may well have that, too. In any case, it's very easy to tweak out.
In spite of having a fair amount of compression the MiSi actually sounds pretty natural live. Enough so that I never realized how much it was compressing until I bought a new condenser microphone and ran some recording tests with the microphone in one channel and the pickup in the other. Then the compression was pretty obvious.
For recording I prefer passive under-saddle pickups. They have much less compression and give a more natural, open sound. However, they are susceptible to noise (arm brushing against the top, inadvertent finger tapping, etc.) and feedback, so for live performance the under-saddle is the clear choice for me.
John