I doubt they're mutually exclusive, in any case. The rosary (speaking of our beads) is a tool. Therefore if we can find multiple devotions to say on it, and doing so doesn't make us neglect the Rosary (the prayer), then hey... why not?
As a devotion... I think there can be no question that any (true) thoughts we stir up in ourselves in regards to the divine mercy are going to be good for us. We live in an age that is already very bad, and that is getting worse all the time. As everything Godly is purged from society and the world around us, it becomes increasingly more difficult to have feelings or thoughts of anything other than darkness or despair.
I think it can be rightly said, that in this day and age, the sins the average Catholic is committing or has committed are probably ten times worse than those committed by Catholics in a day when the average person lived on a farm or something, and didn't see enough occasions of sin to be that bad, unless perhaps their will was to be bad. We are bombarded by occasions of sin 24/7. Many of them we could not escape if we wanted to, short of going into the wilderness and locking ourselves up in a dark cave somewhere. That being the case, it is now easier than ever before for a person to look at themselves and think themselves so rotten that "it's impossible to be saved".
So I think meditations or devotions that recall to our minds God's mercy and love (which are every bit as real as His judgment and justice), are probably more important now than ever before.
In a broader sense, too, Divine Mercy is needed today, not only for ourselves, but also for the family members, friends, acquaintances, etc., who we are surrounded by, who either never knew Christ, know only some erroneous idea of Him or His religion, or who did know the truth, and have since abandoned it (often because of either love of sin, or else some error in their minds).
I admit I'm not particularly familiar with the Divine Mercy devotion said on the rosary, but... I am sure that, as I've pointed out... anything related to the mercy of God and obtaining it, whether for ourselves or others, has got to be a good thing (of course, as long as it isn't offensive to God, or in error in some way.) If it's an approved devotion, it's probably just fine, and a good thing.
But I think it should be remembered that we must be prudent, too... There are countless good and holy devotions out there... but we cant' say or have them all. If we have to many, it becomes impossible to have the ones we DO have, well. If I have 49 other prayers to say, there's no question that the rosary is going to suffer, because I also have a life that I have to live day by day, duties to fulfill, care to take of myself and perhaps others, etc.. We are only human, and so our energies and ability to be devout is limited. Then we get worn out, tired, distracted, etc.. So the thing we need to remember in connection with devotions, is that we should pick a few and just try to give them our best.
If it is better to say one Hail Mary devoutly and with much love and attention, than it is to say many rosaries hurriedly and distractedly, it also stands to reason the same is true of any other prayer or devotion. It is better to say a few faithfully, and say them very well, or to have our minds and lives devoted well in a few things to God, than to say many prayers badly, and have a hundred "fake" devotions (just about in word only), to which we can give no fervor at all, because we have so many of them.
But speaking of the devotion of the Divine Mercy in itself, I'm sure it's quite good.