I would think it's the offenses since no one gives away any real gameplans during preseason. Then defenses adjust during the season to counteract the offenses. Then offenses adjust during the season to counteract the defenses counteraction.
The old saying is that defenses are ahead of offenses early on but you can't prove it by me. Anyway by the first real game of the season we are no longer in the early going at all.
The saying is offense is always behind defense in the beginning of the season.
It is much easier to gel the defense together because basically all defensive players have to is learn their responsibilities. Which gap/technique or what coverage area they are responsible for and what adjustments to make in response to the offensive formations etc. From that point it is merely flowing to the ball or your coverage area etc.
Much of what the defense has to do can be learned in a classroom setting and through film study.
With offense you have the same thing but the added crucial element of timing and precision. That is the major difference! Timing can not be taught in a classroom or even in half speed drills. It is something that can only be accomplished through repetition and fine tuning. If timing is off by even a fraction it will cause passes to be incomplete or runs to fail.
Most people have no idea how much timing makes a difference in everything on offense. Even the center quarterback snap exchange is a function of timing. Ever see a team have to put in a new center during a game and notice their plays (especially run plays) are not as crisp. It could be because the snap is coming at a different time than what they had become acclimated to during practice and during the game to that point. Ever see a RB struggling to gain yds and another RB comes in and has success? Often its because the other RB is hitting the hole with different timing and that is syncing up better with the timing of the blocking.
And don't get me even started on how important timing is in the passing game. Especially the comeback and fade routes.
Even on my youth league team (10 yr olds) timing is the most worked on aspect of the offense. Last year we had to adjust the way we ran plays in a post season tournament because our starting center was not there. The back up center's snap was a tad slower but that caused the QB to open up to the RBs a tad later which caused the handoff to be a tad later which lead to fumbles or RBs missing their hole.
So because of the timing and precision aspect it usually takes longer for an offense to gel together because they have to all work in unison.