I would expect some differences in the surface compositions after treating with oxygen and argon plasma. 

If you already have an argon plasma setup, all you need to convert it into an oxygen plasma is an oxygen tank. You can hook up the tubing to the oxygen tank, and bingo,  you get oxygen plasma.

As already said the change of processing gas should not be a big deal.

However, in my opinion it is not necessary for the first try. Plasma treatment induces formation of free radicals on the PDMS surface no matter what processing gas you use. When your plasma-treated sample comes into contact with air, i.e. when you open the plasma chamber, the radicals react readilly with the oxygen and form polar functional groups CO, OH, COOH. So no matter what plasma you use, at the end you obtain the surface modified in more or less similar way.

I agree to what was said before. However, there may be a significant difference in etch depth and surface roughness, as the sputter rate of argon should be much higher compared to oxygen. Especially for thin films this could be critical...

I expect considerable differences in application of oxygen and argon. Main influence in oxygen plasmas is ensured by O and O- radicals. Inert  gas Ar  plasmas does not etc the surface.

@ Vladimir Lvovich Bychkov: What exactly do you mean by "Inert gas Ar plasmas does not etc the surface."?

I think so. I have not heard about etching by Ar (inert gas) plasmas. But if you mix it with oxygen then it will etch.

@ Vladimir Lvovich Bychkov: Ar-plasma etching is a standard process for thin film deposition (sputter etching), dry etching and surface modification. The high mass of the ions - in general - leads to an efficient removal of material at their impact. When oxygen is used, the etch rates decrease (due to the lower ion mass). However, oxygen is a reactive species, so the etching happens not only due to a physical effect.

OK, regards