The Fireball is 100 percent balls out metal, 6L6 tubes hit very hard. It has a mean voicing that Metal players really like. It only does one thing very well. It does have crystal-chimey cleans but I don't use it for that. It's not a Marshall crunch. Very high gain tones.
The 6505 is much more usable for you non knuckle draggers.
Cleans are nice, lots of oomph but also very articulate and very tight. Very Strong mids. It's a metal players dream, but also can be useful in any genre. Some of them can be hissy of the gain is way up. Gains from low to crushing. (Also comes in a mini head that lacks only the big low end punch if the big ones)
I've played a few SLO (and clone circuits) amps before, I found them to sound nice, with good rock tone, but they are pretty sluggish to me. I'd take a 5150/6505 any day over the SLO. They have more scoopy mids. Moderate gain levels
Just my 2 cents.