I've used both a lot and it comes down to what you're doing.
The rotex 150 is a fantastic monster beast of a sander and can remove a lot of material quickly with coarse grit abrasive and can also produce a pretty fine finish when used on random orbit mode with fine abrasive. You can also polish with it due to the variety of heads available which overall makes it a fantastic all rounder. HOWEVER it does have its drawbacks, it's big and heavy! This isn't really an issue when you're demoing the sander on a piece of wood clamped to your bench but believe me you really notice it when you need to sand some iffy plastering/filler on your walls or worse your ceiling!! Before someone pipes up and says festool make a specific tool for sanding walls, yes they do but a number of people can only (just about) afford one sander let alone one for each task!
So very good but not the easiest to use (as the power gives you a fight to control it), I'd recommend though as it's so versatile and with practice you can achieve great results on numerous different tasks.
Ok now the mirka, there's very little to choose between the mirka deros and ceros but personally I prefer the ceros. The ceros is fantastically light and easy to control, you can even sand edges of boards with finesse that would be basically impossible with the festool rotex. The mirka are really fantastic finishing sanders and that's where the.comparison falls over as the festool rotex is not really a finishing sander. The festool model that's equivalent to the mirka's is the festool ets ec 150. Now the festool ets ec 150 is a very close competitor to the mirkas and the Rep is right, they are pretty much on a par with little to choose between them.
Personally I'd choose the mirka ceros as it's so easy to control and because it's light, you can use it for hours without your arm falling off!!
To summise, decide what you'll do most of (fine sanding or monster material hogging off) then pick your weapon.
Alternatively buy both the festool rotex and the mirka ceros and you'll have the best for any scenario.
Regarding abrasive, definitely definitely buy abranet sanding discs, they are the mutts nuts and I don't care what anyone says, they are the best thing for sanding since sliced bread (that analogy sucks but you know what I mean). Buy abranet and birds will sing, girls will swoon at you, the world just makes sense.
I hope that helps