We just got back from the area... Pretty much what CG42 said above. We didn't do either this time since we'd taken the Maid of the Mist trip before (when they were on the Canadian side) but it's an experience you'll never forget. Pricing is comparable, Maid of the Mist works out to be a couple bucks more at exchange rates where they are now. The USA side while still nice isn't as developed. It's worth a walk over for the different perspective if you have enough time (park area is quite nice, view from the bridge across is great) but if your time is very limited you might choose to spend it on the Canadian side.
Side note, if you have a car probably the biggest tip I got is to make use of parkade at Fallsview casino if you/your co-travelers don't mind the walk to your hotel. You only pay when you take your car out and on weekdays it was only $5. Depending on your dates it's conceivable to park for 4 days for a grand total of $5 (not sure if that might go up as you get closer to peak tourist season?). Price was jacked up to $30 on the day we left (Saturday)... Taken over the whole length of our stay that's still a far cry from the $30/day our hotel wanted and money we felt was better spent on a quality dinner at one of the better restaurants in the area.