I've used both types - a Sunbeam Mixmaster (rotary) and Kitchenaid Artisan (planetary).
My favorite for batters was the Mixmaster. It was easier to add ingredients while mixing and was more complete at mixing than the Kitchenaid.
For bread, though, I prefer the single bread hook of the Kitchenaid more than the two bread hooks of the Mixmaster. In terms of speed and power they are about equal. The Mixmaster does a good job at kneading but needs to be monitored more because the rotating bowl can wobble and sometimes the rotation stalls... both situations easily correctable by caressing the bowl but one needs to be standing there watching during the entire process.
The Mixmaster didn't have a splatter shield, but I don't find them useful anyway.
I've only used metal bowls and prefer them to glass. I once saw a glass bowl from a vintage Mixmaster hit the floor. It was a mess of glass shards. My metal bowls have hit the floor with nary a dent.
For butter, I've put refrigerator cold butter sticks in both and started mixing at slow speed. They "chunk around" a bit but soften up in about 1 minute. The consistency is better than you'll ever get by microwaving to soften butter.