Shine,
It's nice to hear an opinion from someone who has actually owned and sailed both types of Macs over time, I assume in varying conditions . Before buying my Mac22 I wrestled with which type to buy for a long time. Having owned and raced 3 different one design class boats in my younger days (much younger) off the shores of southern Ca. with good winds and ocean, I was attracted to the swing keel and its ability to provide a sailing experience I remembered. However, the attraction of not trailering the weight around was appealing too, as well as being able to maintain ballast while retracting the center or dagger board in the shallows.
I almost went the water ballast. I ended up with the Mac 22 partly because of price but mostly because it provided a very trailerable and easily launched boat (the beauty of the Mac) while still offering that lowered center of weight. I am happy with the decision. I obviously just cruise now but I do enjoy the heavier winds (sometimes to my mates terror). To feel the power of a wind gust and the boat thrust forward without being overly concerned about paying out the main sheet is a thrill.
To be fair, I have never sailed a water ballast so I could be wrong but it does make sense that lowering the center of weight 3-5' below the water line, as oppose to at and just below the water line, provides for a more stable and less tipsy sailing experience in gusty winds (the fun stuff!). With most everything there are trade offs, it just depends on your priorities. And getting back to the original question the gentleman asked, if you are going to sail in a very shallow lake most of the time, and if you buy into the idea that its not a good idea to just partially lower your keel, maybe a water ballast would be better for you if thats a possibility.