The Volvo S-80s I get at SEA are not the T5 Turbo models, but come with the standard 2.9 liter V6. At 190hp, they are surprisingly peppy, but not in the same league as the 252hp Lincoln LS.
While the LS does come in a V6 option, I have only seen V8s at Hertz.
Also in the LS' favor are a six-disc CD changer (Volvo is single-disc), plus heated and cooled seats. Let me tell ya, those cool seats are nice for warm climates. The Volvo has heated seats.
I do fit in the Volvo a little better, but for the same price, the LS is the better value.