I have now played all MM primaries to around level 20. Highest is 22. I use a certain template for them all which uses FF as secondary. I play with an aggressive style where I use my attacks all the time and rarely micromanage the pets. Below comments may not fully apply to MMs who purely support their pets as it is a very different style of play.
All things I write are pure opinions sometimes supported by hero stats data (which I dont report

) So you believe them or not but I have no intention to show any calculations or anything to back this up
My experiences so far...
MMs can be divided to 2 groups where Robots and Mercs form the "Mostly Ranged" and Necros and Ninjas form the "Mostly Melee" group.
Playing Mercs and Robots is a lazy job since they mostly shoot from distance and they both have great survivability so they go on like a train and you rarely see the enemy face to face. In merc set your own attacks have longer range too.
Playing Necros and Ninjas is more intense since they often go melee and (surprise surprise) die often. Gaming is really quite arcade compared to previous group since you have to kill fast or be killed fast.
At level 20 I would rank the primaries by (burst) dmg
1. Ninjas
2. Necros
3. Bots
4. Mercs
By survivability
1. Mercs
2. Bots
3. Necros
4. Ninjas
Problem with Mercs is subpar dmg and leveling speed. Because of survivability you can still kill everything but it sometimes takes loooooooong time. Commando and Serum may totally change this since Commando and especially Serum pumped Commando is possibly the best killing machine in the pet sets (might be better than Assault Bot...sue me if Im wrong

) After level 26 and 32 your damage output and relative leveling speed probably jump more than in any other set. If you slot some range to your attacks you can shoot enemies from a really long distance (fun in PVP :P)
Problem with Ninjas is subpar survivability (and endurance management problems). Ninjas can produce great burst damage
and wipe one group of enemies super fast but they cant keep it up because they have to be healed and resummoned all the time. You also have to rest endurance all the time. Sort of like the "stalker" of the pet sets. A tactical set.
Ninjas are most fun to watch in action though

Jounin is really nice and the first tier pets do nice acrobatics too. Im not sure what Oni will bring to the table...hopefully some more survivability and cool effects

Fun set anyways.
Necros have survivability problems too (Zombies die easily) but it seems not to be as serious as with Ninjas. And the MMs own attacks are probably best in this set. With all the primary attacks and with hasten I can actually produce pretty awesome one target dmg. So what if the Zombies die in the middle of the battle...me and Grave Knight can still kill supa fast :P
Necros are possibly the fastest levelers. I really like this set. Lich doesnt add too much dmg to the set though..it will bring mostly control I think (??) and perhaps add survivability to my zombies which would be nice. And ghost...I guess it is useful too
And Bots...yes they are good but not supa hypa good

Before level 12 they dont level too fast. Protector Bot is good addition and after this Bots and Necros compete with the fastest leveler title. I will run a hero stats test later to find out who can get most xp/hour by hunting
Even though Bots damage imho is not that supa so far they dont die often and can sustain the damage nicely. I often dont have to summon new bots at all in short/normal missions. Bots are great at hunting and in missions where you dont have too complex maps or narrow tunnels. Assault Bot seems to be great and after this Bots might outdmg all other sets and level even faster than Necros.
My biggest problem with Bots is the sometimes annoying sounds and the fact that everything seems(??) to happen in slow motion. Summoning is slow. They shoot hard but not too often. The bots even move quite slow. Maybe it is just my imagination...Effective and easy to play but fun??? I guess it is fun to get good xp and not have to summon all the time

But the sound of my first attack is a real killer..I often turn the sound completely off..playing Bots always makes be yearn for the sweet silence of the Ninjas
I report more experiences when I get them all to 32...which might take a while