Federers decade slam winners: Federer 15; Nadal 6; Agassi 3; Safin 2; Hewitt 2; Sampras 2; Kuerten 2; Ivanisevic 1; Johansson 1; Ferrero 1; Roddick 1, Albert Costa 1; Gaudio 1; Djokovic 1; Del Potro 1
Djokovics decade slam winners: Djokovic 15; Nadal 13; Federer 5; Murray 3; Wawrinka 3; Cilic 1.
The difference is that while Federer overcame a higher number of champions, he faced them for a short period of time or either did not face them almost never....like aging Goran, Sampras, Kuerten or Costa....he clashed for like 3 years (2003-2005) with Agassi....he overcame his generation, which also did not lasted long (Safin, Hewitt, Nalbandian, Ferrero, Roddick) and then he clashed with the new generation (Nadal, Djokovic, Murray, Berdych, Cilic, etc).
On the other hand, Djokovic overcame stablished champions for a longest period of time, like prime Nadal (Djokovic won more slams, masters, wtf, overall titles and head to head during the decade against him), overcame an still absolutly competitive federer (10 time slam finalist and winner of 5), a prime Andy Murray, a prime wawrinka (2013-2017 was a beast), a prime cilic, 4 seasons of a healthy del potro, prime ferrer, and the prime years of the likes of Tsonga, Berdych, Nishikori.
To me, the fact that Djokovic won the same amount of titles (61), amount of slams (15) amount of WTF (4) as Federed,and also won more masters 1000 and finished 5 times as year end number 1 put him above the Federers decade but also, to me , is the best 10 years period ever produced by a tennis player.