Instawallet and Rushwallet have nothing to do with each other expect for being instant wallets. I think you misunderstood that they are related in some way beyond that.

RushWallet is a product of KryptoKit and is more secure than InstaWallet in two specific ways:

1. Funds are stored client side, not server-side.
2. The private key is stored in the URL after the hashtag. By HTTP standards, that part of the URL is not sent to the server when you make a request; it remains purely client-side, so the server never sees what any of the private keys are.