Of course the Blu-rays are better.
A 1080p Blu-ray is always going to surpass a blurry interlaced 480/576i DVD. The Blu-rays bring out the full detail in the 1080i masters that were used for the DVDs (and most of the Blu-rays unfortunately) and even have to compensate by upconverting the picture to fit the screen's resolution.
I think there is an argument if you're looking at them in context though. The DVDs are great DVDs, while the Blu-rays are shitty Blu-rays. TMP is fairly alright, it does have a little too much DNR and scrubbed grain to my eyes, but I'm okay with it. At least it got a new transfer, 2K I'm guessing. TWOK is great (as it got a new 4K transfer), other than that awful blue and yellow tint that's added to make it look more like a modern film. The Director's Cut Blu-ray has even more detail and contrast with a newer 4K scan and is much more accurate to the original look of the film. Both are improvements over the DVDs in terms of picture/audio.
Everything else is a mess.
TUC looks abysmal. Everyone's faces look weirdly smooth and artificial (the closeups of Spock during the Starfleet Headquarters scene look like his face is melting with all the noise reduction added) and there's literally no natural grain to be seen. The DVD copy looks pretty great upscaled on my laptop, I haven't tried it on my TV, but I like it very much as it doesn't look like a video game unlike the Blu-ray. TFF looks meh for the first third or so, but once Sybok takes the Enterprise, it's as if the grain is completely turned off. It makes for a truly strange experience and really gives TUC a run for its money. TVH looks bad, TSFS looks bad, GEN looks bad, FC looks bad... you get the point.
The sad thing about it is that the Director's Cut Blu-ray of TWOK shows that Paramount can put out a good transfer if they put their mind to it. Unfortunately, they don't seem to want to do that.
I managed to grab HD TV caps of all nine films excluding NEM. They're essentially the Blu-rays, just with less detail and less digital enhancements. They're pretty great, but difficult to attain. There's also a 4K untouched 35mm LPP scan of TSFS on MySpleen, which, if you're into the analogue look, is pretty fucking great. It's the only way I watch it now, but it's not for everyone. After seeing it, the Blu-ray looks way too bright, artificial and vibrant.