The Cadillac has a 23 hp and 8 lb/ft power advantage, but also weighs over 300 pounds more. The LS has the added benefit of the 5-speed close ratio tranny. They should be really similar in performance; the LS may even be faster.
The DeVille is over a foot longer, and presumably has a bigger back seat. The LS back seat isn't bad, though, after you squeeze through the door.
The DeVille has a bigger trunk. I'm really happy with the trunk in my LS - I guess I can only fault it for being a little shallow (i.e., not tall).
Finally, the LS has rear wheel drive and a nearly perfect weight distribution. The Caddy has all its weight up front, and it just wouldn't handle like the Lincoln. You'll also feel the road more, but the LS suspension does a great job on the highway and is never overly harsh - though sometimes it can get close. I'm happy my LS only has 16" wheels.
I always fancied myself a luxo-barge guy more than a performance-car guy, and when I bought my LS I did have some concern about whether or not I'd like the ride. As it turns out, I love it. Yes, every now and then on a bad road it can seem firm, but the other 99.99% of the time I'm at the wheel it's damn near perfect.
I have ridden in one of those Caddies and I know they are nice cars, but I would recommend the LS. Big surprise, I know...