I understand the Pensil 11 and Horn XL use 6.5” drivers and the Horn Mk3 uses a 4” driver.
P11 is a 6” driver. FHXL was designed with specific attention to the 5.25” A10x.
FH3 typically has a 4” - 4.5” driver. but we have squeezed a 5.25” in. Probably better in FHXL.
FHXL and Pensil are both good, designed by Scott. The FHXL has greater bass gain, goes marginally lower, and is a more interesting shape. Pensil has a more compact footprint.
If you are working with one of Stefan’s flat-paks, the difference in assembly complexity is minimal. The FHXL does have fewer bits to it (largely due to the frame needed at the back to screw the back on).