I don't think the H&K/Umarex MP5SD is out yet. A lot of websites list it but all of them are backordered. I didn't see it at the gunshow a couple of weeks ago either.
I have the H&K/Umarex MP5A5 .22LR and like it a lot. It shoots just as well as my GSG-5 but I think the H&K/Umarex has a slight edge in build quality.