I have no experience with the HK MP5SD or the MPK. However, I did have a HK MP5A3 (S&H conversion; no 3 shot burst) and a HK SP89. The SP89 was, of course, semi-auto, and being a pistol I wasn't allowed by law to use a front pistol grip. My MP5A3 was a lot more controllable and accurate, even on full auto, as opposed to the SP89. I'd recommend the MP5SD over the MPK. I would imagine that this is not a real HK MP5SD, because the price should be more like $25,000.
Oh, and whichever HK you get, buy extra parts. Specifically the roller retainer plate (that flat wafer piece of metal that holds the rollers in place). It's the only part that I had break on my MP5A3. I received some CQB training at BORTAC and noticed that one of their MP5's also had the same broken part. The MP5 will continue to operate with the part broken, but, I just felt very uncomfortable with a broken part in my weapon.
http://www.hkparts.net/shop/pc/HK-German-Roller-Retainer-Plate-For-Rollers-90p65.htm