In real life, i believe the 74 was chambered for the smaller 5.45x39mm round, as opposed to the AK, which was chambered for the 7.62x39mm round. the 74 was supposed to be the next evolution for the AK series. I personally prefer the 74 over the 47. I think the 74 has a longer barrel than the 47, but please dont quote me on that. im not to knowledgeable on the AK series. I personally like the AK-74 with wood handguards and folding stock.
For airsoft, other than barrel length, the guns are identical internally, with the same style of mechbox and hop-up unit etc etc. Only difference between the two guns airsoft wise is external appearance.
Here is some RS info:
Ak-74U (the sub machine gun version of the AK-74)