Haven't had both, but I did own the P30sk. It was a fantastic gun. Very good quality and the most comfortable grip I've ever had in a handgun. I had the V3 so a decocker and no safety. I loved the gun. The only problem was the hammer hit my hand every time I fired and rubbed my hand raw. It got old. I have lost a lot of weight since then so it probably wouldn't happen anymore. Wish I hadn't gotten rid of it.
OP, what do you currently carry?
Do you currently carry a DA/SA firearm? If so, then I'd lean you towards the H&K. If you currently carry a striker fired firearm, I'd lean you towards the M&P.
With the M&P you'll find more aftermarket support, and much better availability and pricing on magazines. Because when you add in 3-4-5 extra mags to have with a carry gun...that can get expensive.
But out of your two options, the P30sk is great. Have you had any experience with the LEM versions of any HKs?
HK's are like old girlfriends, beautiful, smokin hot, always did what you wanted and you will miss her... Walthers too.
I loved my M&P 9c, but it had one glaring flaw that bothered me.
If you didn't have a pinky extension on the magazine and ran a flat bottom compact magazine, my ring finger would constantly slide off the grip.
Just wasn't enough grip.
In came the Shield 9mm.
Never handled a M&P. As others have stated the P30sk has everthing going for it. My only (personal) issue is the reset. Its crisp but longer than a Walther P99c which is about 1/10". If you are not dead-set on SA/DA ala HK give the Walther P99c AS a serious look. You might be surprised.