Both are technically equivalent. As far as the Rules DSL is concerned they are identical.
The underlying Xtend language provides a little “syntatic sugar”. For method calls that take a lambda as it’s final argument, you can put the lambda definition outside the parens.
I don’t like this and recommend against it in the OH context because most people don’t realize they are creating a lambda Object and passing it to the createTimer method in the former case, whereas in the latter case that is made just a little more explicit since you are putting the lambda definition inside the parens of the method call.
In short, like Vincent says, the later example makes more sense to more people as it is more consistent with the rest of the Rules DSL.