I have a gen 1 MPX SBR, AA94 SBR, Z-5P SBR, SP-10 SBR and a home brew AR9 SBR.
The Sig and AR9 have AR15 familiarity. They are also lighter. They have better accessories compatibility. The Sig is executed very well and is quality...but and this is a big one...Sig screwed all of us that took up the gen 1 platform. Sig switched the platform quickly to gen 2, and has since removed gen 1 caliber conversion kits, which is the beginning of the end of support it seems.
On the HK clone front, I have 3 in 9mm. The Atlantic and Zenith are excellent examples of clones. They are heavier than the AR platform and they are usually more expensive on a parts basis. They also have limited co-wittness optics, but overall, Ill take a rollerlock hk platform any day over the ar platform in 9mm.
With that said this doesnt hold true for .223/.308. I also have two C93 (HK93s), and an V51 SBR (HK91) and a slew of AR15/AR10s. The larger caliber guns are lighter and have better ergonomics in the AR platform over the HK rollerlock.
So in summary. I have and love my MPX, but will never buy another sig product based on the gen 1 experience. But even as such, the MP5 is hands down still the best medium range subgun platform I would go to any time.
Actually everytime we go to the range, im requested to bring one of the 9mm HKs, that has yet to happen for the sig.