There are a couple of noteworthy things. 1) The five year notice is decent under the circumstances. 2) It clearly gives a data point on the time frame and trajectory on the future. Most companies no longer want the long term liability of pensions.
Regarding bankruptcy. Pensions are "guaranteed" under the government's Pension Benefit Guarantee Corporation. I suggest researching this thoroughly. My understanding that the PBGC does not guarantee pensions at 100% if the company goes bankrupt and sells off the pension. It could be as low as 65%.
You must consider whether the 3M will fund the pension for the duration of your lifetime. If you think it could be reduced, then you should consider the lump sum. Note that with the lump sum it will change in accordance with interest rates, specifically the 417e segment interest rates. Generally, a 1% increase in the 417e segment rate from November to November results in a 8 - 12% reduction in the lump sum.
There is a lot to consider. Speak to a trusted financial advisor to make the best decision for you and your family.
The best take I've seen on this so far. Reposting from @jsn+1qtADZ0S for more visibility.