Grip sizes should be pretty much the same for all manufacturer's in terms of the circumference of the handle (which is essentially what "grip size" means). However, grip shapes vary from brand to brand, or even from model to model of the same brand. 98% of the time you should buy the same grip size.
I also play with a grip 3 Wilson. And believe it or not I've also tried my friend's Babolat AeroPro Drive Cortex. I like both grips very much. I'm not sure if it means anything to you, but hope it helps.