The bar diameter is only one factor that affects teh final stiffness. Look at the length of the arms and how they are attached to the chassis and suspension components.
Either way, they would have been designed as a matched set, so my advice is don't mix brands.
Really you only need one of the bars to be adjustable, ie use one to trim it. Having two adjustable bars just gives you more to play with. If they are well matched you shouldn't need both adjustable.
If you are uping the spring rates of your coil springs, you may find that uber big anti roll bars are unnecessary. I would put your springs/shocks in then see if it needs extra roll stiffness.