It depends what kind of guitar music you want to play and to what degree of complexity and perfection.
I'm a classical trained guitarist, and after 6 years, while I'm happy with my progress, it is much more demanding and difficult to play because of the extra strings, bigger size, but most of all because classical music is much much more complex. Most of the time you are playing 3 different voices at the same time.
Even casual playing demands a bit more because chords become more complicated and the extra two strings add more complexity to everything you do from a simple 3 chord song to playing Bach. By the way if you want to learn to play classical or even finger picking, I suggest you find a qualified teacher and take weekly lessons for at least year. The foundation and technique needed to understand the subtleties of this type of music are a basic requirement for the most efficient path to playing and\or mastering this style of music.
They say it takes about 10,000 of practice to master something. well that is certainly true for classical instruments, and unfortunayle I only have about 3000 hours logged in
I would not recommend a smaller guitar, I find it actually harder to play because it's too crowded, yet I can play from an Iuke sopranino to a baritone uke with out any problems.
If you want to check out some of what I have accomplished in 6 years of classical guitar vs. 3-4 years of casual Uke playing I invite you to check out my youtube channel at
http://youtube.com/user/olarte99/videos
IN any case all string instruments have a lot of common so that makes things easier, but out of all the ones I play I find them in varying degrees of complexity from Uke being the easiest, to classical guitar pretty demanding and Violin totally demanding.
For me I decided to only play classical on guitar due to it's demanding nature, and everything else (including a bit of classical) on the Uke.
The benefit of playing both is that my training in classical guitar makes it much easier to play finger picking and melody lines on any uke, and my hands and fingers have developed independence (the bible on this is a book & dvd called Pumping Nylon) so I feel quite comfortable playing any size uke and stretching my hand up the fretboard....
But I have fun with all of them and vary the degree of focused practice needed for them and for the kinds of music I play.
Have fun and Good luck.