Everything ColinsMum said. My son reads both side by side (sometimes literally!). LoF does follow a "path" with Your Turn to Play questions after each chapter and Bridge questions after every 5 chapters--these are questions parents may want to assign. Assigning them though took the fun away for my guy. So we ultimately stopped using the LoF books to teach. They've become reading-for-fun books. MM books are usually free-time/weekend/roadtrip books for the boy.
Given the choice, my son chooses MM over LoF, possibly because I started using LoF to teach while I let him read MM on his own. But this is just his personal choice.
And according to my son (I haven't read all the books myself), LoF does veer towards non-age-appropriate themes in the higher level math books while MM doesn't. Again his personal opinion.
Have fun choosing!
