The older Tamiya kits are somewhat behind the times, as Al pointed out, but the newer ones are a little more accurate (though, true, Al, you have to supply your own feathers! ) Look for their newer T-Rex, for example. The old one is a tripod/tail-dragger, while the newer one reflects more current understanding of how the bones fit together. It even comes with a paleontologist figure, for a "Jurassic Park" dio.
Lindberg also has two generations of dino kits. They have the old LifeLike/Pyro kits, which were fun for me to build as a kid in 1970, but which are really just toys you assemble, like the smooth-skinned Tyrannosaurus or Apatosaurus (fmr Brontosaurus (the Corythosaurus in that series is probably the best), and with serious fit issues. But they have newer kits, developed in the wake of "Jurassic Park" that are better (like a Velociraptor).
Revell-Monogram have re-issued the old Aurora kits, true, but those, too, are more fun for young kids to snap together and play with. If you're looking for museum-quality detail, they're not for you.
If you are looking for something extremely detailed and accurate to the latest theories, you'll most likely get resin, and pay some hefty prices for it. Here is one site where you can see the kinds of exquisite kits are produced today:
stores.homestead.com/.../StoreFront.bok
or you might consider buying finished toys, which, while being toys, are relatively well detailed and can be repaintined or modified. Museum gift shops often have some nice examples, and here is another website for a seller who carries various lines:
www.dejankins.com/.../About_DeJankins.html
I also used to browse Link and Pin Hobbies, but that seller appears to have closed up and gone out of business.
Hope that all helps!
Brad