Depends a lot on how much travelling you do on LH I guess,
SEN status gained via the convential route is valid for 2 years. So if the cash offer is the same that's 750 EUR / Year.
Requalifying for SEN via the convential route gives 2 eVouchers, which are good for either 2 European Y to C upgrades, or 1 routewide upgrade from Y to C or C to F. If that's included in the cash offer how much is a one class upgrade worth to you.
So called 'quiet zone', well unless there are babies. LH reserve the first few rows of 'Y' for SEN / FTL plus I believe exit and bulkhead rows.
Seat blocking, although not guaranteed it's normal to have the middle seat free in 3 x 3 config unless the flight is completely sold. Flying to and from Germany in my weekly commute I seldom have to sit next to anyone.
Very occasionaly the FA address you by name, I like when they do that.
Catering is better in SEN lounge than Diamond Club.
but overall unless you fly a lot on LH not worth the money and if you do then SEN isn't so hard to achieve. Just wish Miles and Less did cash + miles though...