I would like to understand what causes the time difference between
moonrises on succesive days that can be as short as 25 minutes in fall
and as long as 90 minutes in spring. If it is due to only the
inclination of the earth´s axis, please describe why the sunrises do
not suffer from that same effect.
Thank you !

The time of moonrise is affected by four things:

1) earth-sun position (earth's orbit)
2) earth's tilt
3) earth's rotation
4) moon-earth position (moon's orbit)

which means you have, effectively, four asynchronous cycles.

The sunrise is obviously not affected by the fourth.

That is why you see large changes in the moonrise but only small
changes in the sunrise.

Sunrises do suffer from a similar effect.

If the moon's orbit lay on the celestial equator (that is, if the moon
was always right over the equator) the effect you describe would not
exist.  However, the moon does not orbit over the equator.  The orbit
comes closer to lying on the ecliptic, but it's several degrees off
that as well.  And the plane of the orbit precesses due to the
gravitational torque from the sun with a period of around 19 years. 
Anyway, the point is that as the moon orbits, it generally moves in a
north/south direction. On the average, each moonrise is about 50
minutes later than the previous one. But if the moon is in the part of
its orbit where it's moving north, and you are observing it from the
northern hemisphere, you will see it rise earlier than it would have
without the meridional motion.  That is, it will rise less than 50 min
later than it did on the previous day.  Conversely, if the moon is
moving south, it will rise more than 50 min later.  The exact opposite
is true in the southern hemisphere.  If you're near the equator, the
moon always rises about 50 min later each day.  There are still
variations due to things like the eccentricity of the moon's orbit (it
orbits in an ellipse, not a circle, and its angular speed is greater
when it's closer to the earth), but they are smaller in magnitude.

As omega pointed out, there are variations in sunrise due to a similar
effect--the sun moves north/south relative to the earth.  But the sun
moves much more slowly relative to the earth--it takes a year to go
through its north/south cycle, whereas it takes the moon less than a
month.  So the day-to-day variations in moonrise time are larger than
those in sunrise time, and the size of the variations change more
quickly.

Juliet.: O, swear not by the moon, the inconstant moon,  
That monthly changes in her circled orb,  
Lest that thy love prove likewise variable.
 
http://pd.sparknotes.com/shakespeare/romeojuliet/section9.html

Joergs --

You may find this question, dealing with the angle of a sunset, to be
interesting in helping to understand the path that the moon takes
relative to the horizon:
http://answers.google.com/answers/threadview?id=299644

Best regards,

Omnivorous-GA