Is the $500 a month fixed or adjusted for inflation? If the pension is adjusted for inflation, I would keep it. If not it is a pretty close call.
Are there any tax implications for you? The max employer + employee contribution to a 401(k) in one year is $55,000. But there are probably special rules for rolling over defined benefit money. I just don't know them.
Here is my analysis on keeping the pension, if inflation adjusted:
1) 75000 compounded at 6.5% (the average real return of the market, but likely generous given today's high valuations) is $96,485.
$96,485 will generate $321.61 at a 4 percent withdrawal rate. But you have access to the lump sum if you need it and there is a good chance the money will be around for charity/heirs when you pass. On the other hand the money could also run out before you pass.
2) With information of unknown accuracy from
https://www.immediateannuities.com (assuming your 51st birthday is today and you are male) the pension is roughly double the best offer for an annuity that takes 75K today has payouts starting in 4 years and adjusts for inflation. It is a bit more than a 25% premium over an annuity that pays out a fixed monthly amount. As a rule, I think annuities are a bad deal but you are getting something substantially better than any annuity will offer you.