OK, the difference bewteen the HX and EX. Here I go:
Since the thread seems to concentrate more on the difference between the head of the Y8 and the Y5 engines, Ill start there.
As mentioned before the Y5 engine operates on only 12 valves, but up to 2500 rpm's not 3000. It does this by only using 1 pair of rocker arms on the intake side while the 2nd intake rocker arms per cylinder remain unused. At 3500 rpm's, the ECU activtes the VTEC solenoid which in turn activates the additional 4 intake rocker arms, therefore running in 'normal' mode afterwards.
As for the straight VTEC found in the EX, it operates on 16 valves normally, but when the engine reaches 5500 rpm's, the VTEC solenoid switches one of the rockers on each cylinder to an extra rocker located on the head and modifies the air/fuel ratio. To keep this thread from getting to long, I'll just say it's tuned for high rev's after that, makes a nice sound, and makes more power from that point on.
Other differences include the size of the front brake rotors, the size of the power brake booster, the aster cylinder, the ECU, location of the catalytic converter, the header, exhaust pipes, and the HX doesn't have a sunroof. But who give a rat's a$$.
The only reason anyone should get an EX over an HX is if they plan on doing alot of power mods without swapping the engine. Reason being that engine and exhaust mods available for an HX are very rare if existent at all. Why you ask? Simple. Look at where the catalttic converter is on an HX and compare it to an EX. On the HX the cat is right after the header in the engine bay. This helps the engine warm up faster from a cold start, but generally makes aftermarket headers impossible to install and useless as well casue the HX header is much shorter that an EX. Also, if you look for mods for an LX with a non-vtec engine, you'll notice the same rarity. An HX engine basically operates as a normal non-vtec engine when it breaches 2500 rpm's. It saves fuel by only operating on 12 valves(3 per cylinder) in stop and go or idle speeds under that rpm.
And that's basically the scoop there. If you plan on staying stock or doing mods that are everything but enine related, get an HX.You can still do wheels, suspension, electronics, body kits, air intake, muffler, but you pretty much have to leave the engine alone as there is almost zero support in the aftermarket world. One can still upgrade the rear brakes to disc and weight between the models is virtually identical.
I know a few guys with stock EX's, and they think I got screwed with my HX, but with the suspension, wheels, tires and other work I've done, it looks better. It's faster when I drive it hard(they have automatics), and only makes 10 less horsepower, not to mention it drives and corners like a beast. Also, I never heard one of them say they got 350 miles on half a tank before either
Hope all this helps and sorry if I ran on too long.