[REAL 1]

I have owned both an FE428 and currently for the past 14 years an FE427SO.

The performance of both are nearly indistinguishable in street usage IMHO in normal usuage. Both provide gobs of torque for a 2400 lb car and rev great.

The FE428PI was standard fare in 427 street cars with some street cars also receiving the 427SO.

The SC and Comp 427 got the 427SO only and as such the 427SO is the "brass ring" for Cobras because of its higher performance potentials, stronger block, cross bolt mains, higher nickel content block etc.... It is also the rarer motor.

Both are great and will be "correct" for a street 427. If you are a stickler for details and aesthetics you will need a 427SO for a comp or SC.

From above to naked eye with the hood open both are indistinguishable from each other. You need to get underneath to confirm whether its a SO by the ribbing and crossbolts.

A well done 427SO will easily cost you a good 1/3 more than a 428.

Whether it's worth it to go for the SO is a individual choice based on many factors.