Internal wise an MP5 uses an a V2 Mechbox, while an AK uses a V3. The majority of parts are interchangeable for the most part. There are a few differences internally such as air seal nozzle for example. Anyways, all MP5s uses the V2 mechbox, minus the MP5K which uses the V3. Alot of people will say that the V3 design is more robust and will out last a V2 any day, but I believe its all a matter of opinion. A well maintained Version 2 mechbox will last just as long as any other mechbox out there.
In the end, get what ever meets your budget and feels most comfy. One of the best things you could do aside from reading is drop by a game and see if you can handle a couple of AEGs to see what feels comfy to you.
"Its only a little bit on fire"