I own both, which isn't very difficult when one is free ;-)
I am a Modo user from 101, and always upgraded to the latest version.
I like Modo a lot, it has a great community. The only thing i 'hate' about Modo is the lack of cool plugins.
No Vray support or a proper fracture tools to name a few (Blender has both).
Both have their strengths and weaknesses. When money is no issue, i would make use of the offer BUT don't be lured towards a tool because of a huge discount. If you can do all your stuff in Blender...why change?
Cheap (or free) doesn't mean it's worse ;-) 3dMax is much morre expenseive then Lightwave, yet i would pick the later any time.

Discounts come and go (although 40% is cheap(ish))