A couple of points to consider:
1. The PBGC (which is like the FDIC for pension funds) will pay pennies on the dollar for pensions it considers large, much like the FDIC only guarantees up to a certain sized deposit.
2. When calculating what you can safely withdraw from an IRA or other investment account, the planning number is 4%. That takes into account inflation, and market fluctuation, with a 95% chance of your asset/account lasting as long as you need it. Average stock returns are higher, of course, but if you take out all of the growth each year, you're losing compared with inflation, and you'll bury yourself in a down market year.
3. Your nearly $500/month now is nearly $6,000/year, and represents a 6% return on the $100,000 they're offering. That's not bad.
4. The nearly $1000/month 9 years from now is an 8% annual return on the value of the pension. Also, not bad.
5. The $100,000 lump sum, invested properly, at market rates, will yield slightly less than what they're offering in monthly pay out. See note 2. above.
6. We've seen phenomenal market gains over the past few years. I can't predict the next correction, but don't forget where we were in the summer of 2008, with some stocks losing 50% of their value in that year. (It was a great time to buy, and I did, but that's another subject altogether). So, in taking the lump sum, you're open to that type of market fluctuation. That type of risk.
7. The real question for you is one of risk tolerance and asset allocation. You may decide, for personal reasons, that you would like control of the money, understanding that you will likely not meet the monthly payout that they're offering. That's a reasonable decision. Similarly, leaving it with them, to have a fixed income/guarantee allocation in your overall financial position, is a reasonable decision.