Check to make sure you can actually do this. You may have to give a certain amount of notice to be eligible, or they may cap it at ____ hours, or some other B.S. fine print.
At my company, I would actually take the vacation. They keep changing the policy (3 years, 3 changes in PTO policy), making planning ahead rather futile. People who saved have gotten more screwed than those who haven't. Caps are reduced, rollover rules are changed, etc. Even at DH's company, they had "forced vacation". So I guess a certain amount of this decision is risk-based. I don't trust people anymore and would just take time off.
That aside, if you're sure it'll work out the way you want, I'd take the payout unless you need/want another vacation, only because you'll be FI after that and it would be a nice bonus. I don't think there's need to take it "just because", but any reason would be good enough for me. I'd take my birthday off and get drunk before noon, for instance. :)