...My first choice is the Streamliner 600, then the TC RH450. I prefer the tone of the GB, and if the STM-600 has enough horsepower I'll go that way.
...How does the STM-600 stack up against the RH450 volume-wise? Do you have to push the TC harder than the GB to attain the same volume, or vise versa? How do "Genz Benz watts" compare to "TC watts"?
...I should say I'm in a loud rock band, with the guitarist rocking a full stack of Peavey 5150's.
...Is the STM-600 gonna cut it for me volume-wise, or should I save up for the 900, or go with the TC?
The only major difference will be how you set the knobs. Genz and TC watts seem pretty comparable, so there should be no surprises there. 375w into 8 ohms and 600w into 4 ohms from the Streamliner 600 should be enough for any reasonable situation. So if you prefer the Streamliner tone, I can't think of a single reason to settle for the TC. Even if you like stuff like the built-in tuner on the RH450, there are lots of good pedals out there...
That said, I just surprised myself with my gut reaction to your question. I'm normally pretty conservative about power ratings (I've played in rock bands with a little SWR Studio 220), and the difference between 600 and 900 watts isn't that great. Also, I originally felt some guilt about buying the 900, when I knew that a 600 would probably do everything I need.
However, after living with the 900 for a while, I can't see why you wouldn't pony up the extra bucks.
For one thing, don't forget that all three of these amps represent a pretty significant investment, and to my ears, the SL900 tore the RH450 a new one. If you were really serious about saving money, you'd have settled for an Ampeg PF500 or a G-K MB500. Since you're not settling, why not go for the gusto?
For another, what's that saying, "buy once, cry once"? If you like the SL tone as much as I do, this could be the last amp you'll ever buy. Given that the price difference between the 600 and 900 is about the same as a good pedal, why settle for second best?
Also, you mention that you haven't finalized your cab choices yet. The SL900 will give you more flexibility in a modular rig, thanks to the 500 watt rating at 8 ohms. Imagine having equivalent power to a maxed-out Markbass LMIII, without being tied to a 4 ohm load!
Finally, the combination of light weight and effortless headroom from the SL900 is pretty addictive. If your guitarist is really opening that 5150 stack up, then there will be times when you could use the extra booty. Of course, you could always pair the SL600 with a separate power amp, but you could do that with the SL900 as well. Meanwhile, you're less likely to feel the need with the bigger head. So why take a chance on coming up short, when the price and weight differences between the 600 and 900 are minimal?
