Late to answer here. Anyway, both are technically good VNs and I'm sure you already tried both. That said, in case another person somehow got stuck in choosing between two VNs, then Amazing Grace should be better choice when it come to saving the money (And HDD), and it has mystery as the bonus. Technically Mashiro Iro is not bad VN by any mean (Especially if you like charage), although it's a bit more expensive compared to Amazing Grace, and not to mention you need to technically buy Sana Edition to call it as complete.