Celotex 50mm or Rockwool 100mm

Hello,
I want to create a sub-room inside my Garage as a small utility room. I may put a PC in there for some office work too. I am planning to build a stud frame dividing the garage into the small room and the remainder of the garage, which will be used for storage.
I will be using 30mm insulation backed plasterboard for the three walls.
I am going to use CLS 89mm wood for the frame at 600mm intervals.
I was planning to use 100mm Rockwool (I'll have to squash it a little bit) but the builders merchants said it would be better to use Celotex 50mm for the walls as it is more much efficient. I know this costs a lot more and it is a little tricky to work with. What is generally recommended?

100 mm of insulating wool (of any type, they have same R values )compressed to 90 mm will give you the same insulation value as 50 mm of Celotex, but have the following advantages.

a) It will be around 40%-50% cheaper ( for roll type, not batts )
b) It will give you some sound insulation

The advantage of Celotex is that the frame can be thinner and so you don't lose so much space in the new room

Well I have already bought the 4x2 and road noise is a concern. I will go with Rockwool. Thanks both!