[tinfoil hat], but the difference between DVD and Blu Ray is amazing depending on the Blu Ray I suppose.
That is the key part, depending on the Blu-Ray, some early ones were basically upscaled from DVD masters thus were effectively no different to watching the DVD on a good upscaling player.
Most/all new ones now, should be based off HD masters if they were available.
I watched Blade Runner Final Cut on DVD about 2 weeks before giving in and getting a Blu-ray Player, and then watching Blade Runner Final Cut on Blu-ray.
A lot of the differences were subtle,small details (both were based off the same remasters, just one at DVD res, the other at 1080p), but there was definitely an improvement on the Blu-Ray version.
Back on topic

As has been said, as long as your cable isn't too long, any HDMI specced cable will do an identical job in terms of picture/sound data transmission over a short run, be it a Poundland special, or a £1000 qold plated oxygen free cable, hand made by Tibetan Nuns of the ancient order of Her Lady of the Idon'tbeleivepeoplereallybuythisstuff.
Over longer distances it can make a difference.
Of course, if you're going to be unplugging it a lot, or putting it in a wall or similar inaccessable place you may want to pay more to get a more robust cable, but robust in itself doesn't mean it's any better at carrying the data in normal use
