both are tools with the same goal, but drastically different.
a crossover trys to blend different speakers together. you would use a crossover on the subwoofer and front speakers to get them to blend together.
an EQ is a general processor, it is used to make small corrections to the tonality of the system. its kinda like a more advanced version of the bass/treble knobs on stereos.
since these are both tools, and because of the issues they address, neither is "better". both are helpful.
a crossover will allow you to highpass the speakers, which prevents them from playing bass (leave that to the subwoofer).
an eq is a general tool and can be useful.
these are tools. this means the "max" setting isn't the best. if you don't want to learn how to use them, you may be better off without them.
Let me make some aclarations
A crossover splits the signal from the amp into low, mid and high frecuency, and directs it the respective subwoofer, woofer and tweeter.
In this way, you make sure that, for example, the Subwoofer will not try to reproduce high tones and blow up, or the tweeter try to reproduce bass and fry inmediately.
Spliting the signal to its correct speaker to reproduce it also improves the quality of the sound, instead of blending all in a single "full range" speaker, typical of the car's factory installations, with poor bass.//content.invisioncic.com/y282845/emoticons/crap.gif.7f4dd41e3e9b23fbd170a1ee6f65cecc.gif
If you look closely, you will see a little capacitor (a cilindrical shaped thing) connected to your tweeters. This is a form of basic crossover, prevening low frecuencies to reach the tweeter and set it on fire. Offcourse, more advanced tweeters are much more complex and expensive, for example the JBL ones, but are precious looking and perfect sound.
The EQ is much more than a bass & treble control. You can make specific changes in the frequencies, so the specific sound of a guitar, drums, or the voice are under your control, and you can improve or get rid of undesired frecuencies, to for example, compensate a bad recording.//content.invisioncic.com/y282845/emoticons/eek.gif.771b7a90cf45cabdc554ff1121c21c4a.gif
Offcourse, a EQ is not magical, you must set it in order of the music you are lisening to. Needs some time to get the perfect curve that fits to the majority of your music and your speakers efficiency. Improving the wrong frecuencyes of puting all of them in full gain (a common mistake) will only cause distortion.
Do not confuse an EQ with the Rock Jazz Blues preset of the new radios, thats a crap.
My EQ is a old school Pioneer EQ-7000, I love it almost like a girlfriend hehehe. I always set it like a smile, with a little gain in 500 hz to improve some instruments or voices. Just personal opinion.