My conclusions, for what they are worth:
FZR600 Fazer. Older bike with carbs. Very predictable, high feeling of weight. Comfy, safe and all-round a super bike. The high end of the rev counter can give you some cackles as it goes like a banshee.
FZ6. Newer, fuel injection. Really, really good bike. Again, high-ish weight and comfy, but after a CBF500 or GS500 (both really good twins, though the Honda is about 20 years ahead of the Suzuki and one lovely bike) feels really revvy and I stalled it like a t**t for the first few miles. However, once you get it up to 7krpm you'll be dribbling, the acceleration is just soooo good, and the noise is brilliant. However, it can be bit tiring having to keep the revs up just to stop it stalling.
SV650 - wow. What an amazing bike. When you put it next to the Fazer it is pretty much the same size, but it to ride it seems smaller with a lower centre of gravity and is way, way more sporty to ride. It actually asks you to lean into corners and is so chuckable. To be honest, I'm really new to this so I'm still getting my head around what a bike can do, and what I can do, but the SV makes cornering really good fun in a way the Yamahas just don't. And the engine is a peach. Enough torque to make riding easy, but when you wind it up it really can shift. Low insurance category too.
To be honest, there is no competition between the GS500 and the SV, but you ought to try a CBF500. They are an underrated bike (except by couriers who love 'em) and the ergonomics are superb.
Anyway, whatever you go for, enjoy.