Are landings easier during the day or at night?
I'm going to answer this in somewhat of a more general sense but first let's clear up a few things.


  During the night the runway is brightly lit and all the guiding lights
  are clearly visible, but the actual tarmac is more difficult to spot.


This is not always the case. Maybe at large international airports the runway is quite well lit with lots of extra lights, but there are plenty of airports in the US that are poorly lit (granted not all of them service commercial traffic); it's worth noting that not all runways are well lit. Runway lighting varies quite a bit as well. 


  During the day there is flare from the sun 


Only if the sun is right in front of you, there are lots of runway directions and you may often land with the sun behind you. 


  ...and the lights are less visible but on the other hand the tarmac is clearly visible.


The lights are not visible at all, they are off, but yes the tarmac is easy to see during the day. 



A "good" landing by a proficient pilot should not be greatly affected by day or night conditions. What will affect a landing is the prevailing wind and weather conditions. Generally speaking wind (and thermal activity) dies down at night so the air is far more calm. In my experience night landings are smoother because of this. During a hot summer day, even in super clear weather, with the wind whipping and the clouds kicking thermals around you are bound to have a "fun" approach.
(from the FAA's handbook on night operations)


  Night approaches and landings do have some advantages over daytime
  approaches, as the air is generally smoother and the disruptive
  effects of turbulence and excessive crosswinds are often absent


More recently LED light technology has allowed some extremely bright landing lights to be installed on even the smallest of aircraft, this has greatly increased pilot visibility on short final at night.