Another internal mic option is the Miniflex model 2. See:
(He's a Delcamp member)
That's a very smooth plugged in sound IMO (no EQ too, pretty impressive).
Generally speaking, internal mics will give you more feedback problems live than external mics and a preamp with good EQ is essential to control problem frequencies and shape the tone (internal mics tend to have a boxy, honky mid range sound to them). Both the Bartlett and the MIniflex are good, and are designed to avoid these issues but there's only so much you can avoid these issues
Having said that, Ken doesn't seem to have that honky sound at all in the above clip.
For an external clip on solution the best on the market is the DPA 4099. I have one and I have also had a Miniflex.
The one thing I just can't get used to with mics that temporarily attach (internally or externally) is the trailing cables, built in preamps which dangle in the way or go in your pocket etc. And at high volumes (eg weddings outdoors, guests dispersed and background noise) they will never cut it. You don't mention the environments(s) you need this for, which is a big issue. Everyone who plays live wants a natural sound, no one wants to modify an expensive guitar. If you will be playing in noisy environments, or a wide variety of environments even, you quickly discover the need to compromise.