On the matter of the covers: I say keep the "tome" style. I like it, and it's different. And, as someone else pointed out, it's setting-generic.
On the matter of internal art: In the best of all worlds, this would be a non-issue. I'm not personally a fan of "dungeon-punk." I don't insist that armor and weapons be copied from a history book, but I'd prefer closer to that than some of the truly outlandish.
But...
Why the heck does it have to be an either/or thing? If you go back to 1st Edition, the various pieces of interior art were of wildly differing styles in some cases. Some people might feel that's a problem, but I think it add to the feel. D&D is supposed to be out options--so provide multiple viewpoints. Do some dungeon-punk pics. Do some that are so historically accurate they could be copies from actual wood-carvings of the time. Do some (most) that fall between the two extremes. Heck, do some for non-Western cultures.
Who decided every picture in the game had to have the exact same "feel"? For games with specific settings, that's fine. For a game that's supposed to be usuable to recreate any sort of fantasy imaginable, sticking to a single style is a weakness, not a strength.