We can not say any of the activity is difficult or easy, everyone has their own choice my personal opinion is on page needs more focus or a experienced guy as it can not be changed more frequently and the other hand off page can be hand over to the freshers or newbies.
According to my way, both on-page SEO or off-page SEO strategy is no more difficult. Because it depends on people workability and capability of processing. So different people use a different strategy for doing on-page or off-page SEO.
I personally like the on-page SEO more, because it seems to me that the results of it depend solely on me, on how well I select the keywords and phrases or how wisely do I insert links, etc. It does not mean that I do not realize how beneficial the off-page SEO is or do not rely on it in my work - but for me, the results of it are always quite hard to determine.
On-Page: Quality Content, Design, Internal Links
Off-Page: Quality Links from other websites, press release submissions. it is difficult to have a good and working SEO strategy for links and it takes time and work.