I own the Canon 100mmf/2.8L macro, and I bought the 65mm MP-E, but sent it back after a couple of weeks. I love my 100mm macro, but personally, I found the MP-E was too difficult for practical use:

• there is no focus ring... you focus by moving the camera. This means a tripod and a focusing rail at a minimum. More gear than I was willing to carry for what I was interested in at the time.

• depth of field is paper thin... this means focus stacking for virtually anything with any depth at all.

• focus stacking means a carefully thought out sequence of images, plus more equipment: image stacking software & focus rail at a minimum. The fine adjustments required on the (mostly) poorly-designed manual focus rails is tedious and error prone for a lot of shots (hundreds might be required for an insect). An automated stepper would help with the tedium, but still requires thought, setup and greater expense.

• how are you to keep those insects still during all of this set up and exposures?

OTOH, you can get some pretty good closeups withe the 100mm macro. I pair mine with a 5DS-R, and that gives me scope for cropping if necessary. I don't mean to imply that focus stacking is unnecessary w/ the 100mm macro, but two things counteract to mitigate the chore:

1. depth of field is (usually) greater, so fewer exposures required

2. 100mm macro has autofocus, which means it can be driven/controlled in an easier fashion. I particularly like the Helicon FB Tube as it is small, light and Helicon's aux software (runs on phone or tablet) helps with automating the shot sequence (Disclaimer: I've not tried this myself, but have seen a friend's results).

Anyway... there are some shots where you just need the MP-E's magnification. If you do, that makes it an easy choice. If you can get your shots with the 100 mm, I believe you're better off with that setup.