I wouldn't agree that the SLO is an apple and a 5150 is an orange.
I'm not going to get an argument wether there is some magic to SLO's that 5150's have or not. There probably is in the room. Plenty of good recordings made with the SLO too. But saying the SLO and 5150 aren't related is simply disregarding facts.
They're not clones, and if one can't tell them apart when having them side by side probably means he is not very experienced with amps or doesn't have very discerning ears, but it's been discussed to death how they're related. I'd classify an SLO and a 5150 as more closely related than not, personally.