Assuming the employer is steady (i.e. you don't see the pension "failing" in the next 20-30 years), I'd take the 500/mo now.
It pays you 6k/yr.. out of 104k, that's a 5.77% annual return. If the pension is stable, that's a higher amount than you'd be able to safely take with a 4% WR ($4160/yr or $347/mo).
You also need to look into the tax treatment of each, especially tax penalties. It's quite possible you could have to pay a 10% early withdrawal penalty taking the lump sum now (before age 59.5), but that penalty won't apply to the monthly amount, which could be treated as equal periodic payments, basically.
A 10% penalty (even ignoring the tax on whatever bracket you're in right now) would knock it down to $93,600, and a 4% WR on that is only $312/mo, much lower than their $500.
The difference is, of course, you're left with the principal at the end in the lump sum one versus the pension, but I'd be taking the 500/mo now, personally, investing it all, and using it to decrease your time to FIRE (both from the extra money it generates now while still working, and from the reducing/offsetting your budget by $500/mo in FIRE, so that's $150k less you need to save--again, assuming the pension is stable/reliable).
Another key question though: is it inflation adjusted? I was assuming yes, but that all affects the above. There's not really enough info for us to give a real decision, I'd recommend running some scenarios/projections in excel based on various estimated rates of return and see when you'll come out ahead (similar to what Cassie did). But you'll need all that info first on tax treatment, COLA, etc.
Good luck!
