The $540 you're paying on sale sounds closer to what the actual retail of the bike should be. Improvements in going from your current 7300 to 7500 would be marginal unless the 7300 is completely worn out. I have a 2001 7200 and would not consider this upgrade.
Are you planning on using the 7300 or 7500 for touring? The low spoke count and suspension fork on the 7500 may give you some problems. I can't tell if there are rear eyelets.
If by "chain coming off" you mean the chain is actually coming off the crank or shifting by itself you should be able to get this adjusted out. The problem that I have with the grip shifters is that I accidentally shift when grabbing the handlebars. Using bar ends should help this and give you more hand positions.