Differences between StringBuffer.append und String +

The Script

class Test {
public String toString()

{ super.toString() + properties }

}
println new Test()

produces:
Test@2a5330["metaClass":groovy.lang.MetaClassImpl@1b383e9[class Test], "class":class Test]

but the Script

class Test {
public String toString()

{ new StringBuffer(super.toString()).append(properties) }

}
println new Test()

produces:
Test@13c1b02

{metaClass=groovy.lang.MetaClassImpl@91cee[class Test], class=class Test}

Is that a bug? I would think this should produce the same result. If the first class would be compile it should generate something like the second class. So this could also be a difference between scripts and compiled code? (I will try this if I got the time.)