For the cost of the HK sear only, you could buy a complete FA Uzi.
Advantages of UZI:
Cheaper mags
Slower cycle rate, about 600 rpms
Robust and handles abuse well
Cheap and available spare parts
Advantages of MP5:
Accuracy over UZI in smei, FA and of course first shot placement. The UZI is accurate, just not as accurate as the MP5, especially at distance.
Weapon platform. Many configurations possible with just one gun.
Balance and "feel." Easier to become "one with the gun." (Sorry for the Zen).
Lighter weight. Very nice when carrying weapon and a ton of mags.
Much better sights than the UZI. I personally dropped a HK91 v notch rotary drum on my MP5.
My advice, for those who can afford it, go with the HK MP5. It is a better subgun, but realistically will cost you more than twice what the UZI costs.
For those who cannot afford it, go with the UZI. There is nothing wrong with the UZI.