Klooney,
If you are in the in the waiting period for the Jazz, dont cancel just yet. The Baleno will launch on the 26th and there will be another month or so before the first impressions of that car start coming in. So you have time till your car get registered in your name to cancel the booking.
The Santro will always be better at hill climbs than the Jazz. The Jazz is a bigger car and optimised for fuel economy. You will need to use a lower gear on a hill climb.
From my experience with my Swift over the last year, every car has its intricacies. Even the same model in different trims behave differently. For that reason, always ask to test drive the same model which you are going to buy before booking. Even then, for an average user, it will be 6-8 months before you get to know the real character of your car
Regarding handling charges, you can negotiate a discount that wipes out those charges but, unless there is an ongoing offer, handling charges are not waived off. By the way, there is currently no offer running on the Jazz which is really sad
Ameyam