True, the 2.13GHz processor is available only on the 256GB model. I'd suggest going for 4GB if you are going to multitask a lot. If you don't need 256GB, then the 1.86GHz 4GB 128GB model is likely sufficient.
And if you really need the CPU power, 1.86GHz or 2.13GHz won't make that much of a difference.