It's a funny old thing to compare this one. In a lot of ways DJing and Producing are very co-dependent and that's why most DJ's are now producers and a lot of producers take to DJing. It's natural to need to promote your own music as a producer and it's natural to want to compose your own music as a DJ. If nothing else curiosity gets the best of you and you at least find yourself trying it. DJ's and producers can't really exist without each other, because somebody has to make the music for DJ's to play and someone has to promote the music for producers to sell.
I've been DJing a lot longer than I've been producing, so I still find producing a lot harder and a lot more time consuming. However there are people that can produce great tracks very quickly and there are people who agonise for hours or days over DJ sets. I've seen a lot of talk of "anyone can mix" and "software does it all for you" and although software is better now, that kind of talk I find completely misguided and misses the nuances.
Back when you were playing live on turntables, you could turn up at clubs with horrendous lag on their sound systems, technical issues and all kinds of things to deal with. Needles might skip on you, or even Tiesto as I've seen happen and your perfect mix is in ruins. That's not easy to deal with. Of course technology has made the basics easier in that way, but now you're thinking more of harmonics and progression. You might be thinking "How can I surprise this crowd or my listeners and bring the energy up?" If you're playing live you might see the crowd react badly to certain tracks and realise you need to change the direction of your set. It should always be about providing the best experience for everyone who is hearing what you're playing and it takes an intelligence, thoughtfulness and skill to do that. I think stuff like that is underestimated.
Also underestimated are the skills of the very best technical DJ's. A good DJ has the ear to break his track up and realise what the bass, mid and treble adjustments will do to the track in the mix, possibly what different effects and now different structural changes might do too. That way instead of getting a bit of a messy sounding, but beat matched mix, you can get a smooth flowing perfect mix as well. All of this can become automatic in time, but it takes time and practice to develop that subconscious level of working, especially in a live setting with different sound setups facing you at every venue.
I find that if I want to just mix, I can put something together that'll work, but to get something I'm perfectly happy with, that I still find difficult and I know I could personally improve so much. There's always room for improvement and if your skill ends with letting a computer beat match for you, then you're not really a DJ or somebody who understands the real process of DJing in the slightest. Look at someone like Zabiela and his skills level in a live setting. There aren't many DJ's, producers or anyone else that can do what he can and that's because of his talent for DJing and his years of practice.
As for producing, well I do find it harder, because my experience level isn't the same. You can never learn enough, never understand enough and never really feel like you've made the best track you can. It's always evolving and you know in six months you'll know more than you do now and you'll wish you could have re-done that previous track, because you can hear every horrible part you feel is a mistake, even if someone else doesn't notice it.
Apart from learning the skills of sound design, synth programming and how to put a mixdown together, you also have to have an idea of how to compose melodies and create harmonies with atmosphere. This might be where the infinite possibilities of music composition might make it trump DJing for skill, because the great producers can produce with not only tight production, but also the melodic flair you'd expect from a classical composer. You always need original ideas and you need to create your own sound.
I'm sure the guys at the top level of both DJing and producing will tell you that both are hard and that if you're not challenging yourself, you're probably failing. I've rambled on long enough now, but I suppose I find it difficult to see DJing trivialised down to throwing a few tunes on and beat matching. It's almost the same as saying anyone can run the 100m. Running is a pretty simple process and a lot of us can do it. It's funny though, none of the rest of us are quite Usain Bolt or ever will be, because we just don't have either the talent, attributes or dedication for it.