Straight cut gears eliminate the axial thrust that helical gears cause. The axial thrust increases with spring tension. If you have BB Chevy springs, or stiff K800s, you pretty much must use straight cuts since the stock thrust bearing (doubled) probably won't withstand the stress. However, if you are using VW sized duals or less, you don't need straight cut gears.
As for the hp difference, you probably couldn't measure the difference on any dyno, it is very slight.
If you like the sound, get them. If money is tight, spend your money on the heads, where you'll actually make more power.
THANKS BRUCE, I REALLY APRECIATE YOUR COMENTS
JOSE LUIS