Boris have a look at this - especially the bit at the end. I know the balls not a baseball or a cricket ball, but the curve it gets is ridiculous!Kram's email just popped up in my inbox (for some reason yours didn't, Dave) so I'll pop back in and see what I can add. A lot of this you've probably either already worked out or have read somewhere, but I'll put down what I know anyway.
I dropped out of physics earlier than most would so I can't prove it scientifically, nor can I seem to find much info on the internet as to why, so this is all personal experience. What I find the main differences are between the two balls are size, weight texture, material, ageing and especially seam configuration.
As a pitcher at club level I rely purely on pace, I can get pitches up around 130 km/h most games based on the few occasions we've had the speed gun out. This is a lot for a club game, but I don't have the accuracy or variations to make it much more useful than somebody slower but more skillful. The few variations I have are all based around the seam, I find.
As you probably know, a baseball's seams are in the same pattern as a tennis ball. You generally pitch either a 'two-seam ball' or a 'four-seam ball'. The number refers to how many seams (or parts of seams, as technically it is all one seam) will rotate through the air with backspin, just pick up a tennis ball and give it a go and you'll see that you can either throw it with two seams rotating through the air (with backspin), or turn the ball around 90 degrees and four seams will rotate through the air.
Obviously four seams will cause a lot more drag, which is why you throw your fast ball with two seams to reduce drag and increase pace, but curve ball, screw ball, change up etc with four seams to increase the drag to increase the amount of curve you can get in the air.
The seams are much more pronounced than a cricket ball so there's even more drag created there. The texture of the ball also seems to feel rougher and grippier in comparison to the slippery cricket ball, so maybe that also helps.
The pronounced seams also make it a lot easier to get physical purchase on the ball. To impart spin a lot of the time you have your finger run directly down one or two of the seams, plus the ball seems to be just the right size for your palm. Both of these I would say much increases the amount of rotations per second and therefore curve, although I don't have any actual research to back that up.
The cricket ball is generally quite slippery and you have only the 'one' seam to grip with, which quite often gets flattened out and smooth by the time a spinner gets to use it. Particularly as a finger spinner (which is basically how all spin is generated in baseball) I find the biggest problem with me getting any turn is getting sufficient purchase on the ball, and I always wonder how much energy is being expelled throughout the action as my fingers struggle to get purchase. Baseballs seem to just stick to your hand and are a few grams lighter.
As Dave said, balls are replaced every time it hits anything other than hand or glove (as much as possible, anyway, as club games can quite often run out of balls quite quickly). This is to both stop visibility issues as the ball goes either red or brown once it hits the ground, and also to stop any wear, otherwise the pitchers will simply be unable to control the amount of curve on a ball and it becomes a safety issue. This obviously means there is no swing in baseball, which is what cricket relies on due to the inability for large amounts of curve.
All this said, baseballs seem to be designed for an incredibly boring and one-sided affair. Despite loving pitching, I don't like the rest of the game very much due to lack of anything interesting happening at all. It always seems that even five-day Test cricket is moving faster than a three-hour baseball game. Pitchers dominate around 75% of play due to the massive help they get from these balls and makes it incredibly difficult for a batter to score. Efforts have been made throughout the years to help balance this a little, such as lowering the height of the pitching mound (another advantage, pitching from a height down at the batter) and allowing advanced bat technology, but it really hasn't helped. In my opinion baseball would be a much more interesting and better game if they used something similar to a cricket ball.
Sorry for the long post, as ever, but hopefully this covers a few things.
Air is a quite magical thing!Boris have a look at this - especially the bit at the end. I know the balls not a baseball or a cricket ball, but the curve it gets is ridiculous!