Maybe APP is more busy than tower, but at least here in Abu Dhabi GND is the busy and difficult one.
We are still mostly dual rated over here, so a bit hard to compare, however, I feel a controller is a controller, and no ratings are superior to other ratings (unless you are south-african, as they all seem to think APP is the superior one.)
If you are all single rated in Doha, then comparing one to the other is like comparing apples to oranges. Who should be paid more? TWR, APP, ACC, dual or trippel rated people?
I am sure in your contract there is a stipulation of how many shifts and hours you should do during a month, and as long as you are within that no need to complain. If the guys with TWR ratings are working fewer shifts then great for them. If you are working more than your contract says, then you should be compensated for it, either with more pay or compensatory days off.
And to compare, I believe the guys at ACC gets paid more than me, and working fewer hours than me, but I don't cry about it...
Out of interest, are you getting paid more or less than the guys working ACC? (I believe it is Bahrain doing your area airspace?)