Are you about to buy one?
If you want to make a comparison between the Super D and a PA18 you will have to define what you want to do with it. They are essentially built for two different purposes, so to define that will be your starting point.
The Super Decathlon is an aerobatic trainer and the PA18 is built to fly low, slow and land/take-off on short, rough strips. You can do that with a Decathlon as well to some extent, but the lack of flaps, and the positioning of the spring steel undercarriage makes it slightly less efficient and sturdy than the PA18.
The Super Decathlon has a somewhat restricted payload. There is an option to increase the weights by changing the gear, however that only affects the utility category and not the aerobatic category. This is probably the main negative issue with the Super D.
When it comes to pure handling, it doesn't have any minus points. It's a really lovely aircraft and very agile. Comparing it to most common aerobatic trainers it will beat most of them. The 180hp C/S with the inverted oil/fuel system lets you do anything. (Outside loops included). Wing profile is very close to being fully symmetric which makes inverted flying really manageable. It's very light on the controls and has a descent roll rate.
If you want anything more agile you will have to go for a Pitts or similar.
I don't agree with Niall that it's tricky to land it. Neither wheel landings or three pointers are difficult. I do about every second landing as each. I wheel it on the long tarmac runway where I vacate somewhere far at the end. But I always three point it at the short and rough grass strip I regularly visit.
I could say a lot more about the Dec's, so please ask if you like! But define your question a little bit more, and it'll be easier to be more precise.