You are in the same place I was about 6 months ago: keep my SV650S or trade-up for a FZ6 or FZ1? Thoughts on each:
SV650S: put about 2500 miles on a 2007 model. Great torque off the line, low rpm operation doesn't get you all wound up. Brakes are great, suspension is so-so, handling was pretty great in the twisties even with stock tires. Fuel economy was brilliant, somewhere close to the 60s. Didn't really like the slightly less (than an FZ6) wide rear tire, but it turn in better. Really hated the numbing of my wrists after about 30 minutes of riding and the discomfort in my lower back. So, riding along one day I stopped at my dealer and...
FZ6: saw 10 of them out there, just sitting. Were being sold for LESS than what I paid for the SV. Asked to take a ride on one, guy throws me the key. Start it up and all I feel is sssssmoooooooooth, unlike the v-twin rumble. Nice... put it in first, CLUNK, and ease off. Stalled it once but not again, and finally got going down the road. After riding on the SV already about 45 minutes, the FZ6 was my new chiropractor. The seating position and angle of the handlbars made it feel so, so, SO comfortable it was like swallowing 2 Advils 15 minutes ago. Almost forgot to double check speed, look down and see 75 MPH on a 45 rural one-lane highway, SLOW DOWN to legal speeds in one second, it seemed like, and try thinking of reasons NOT to trade up. One by one, they faded: insurance would be cheaper than a SV, better financing thru Yamaha, 2 more cylinders and a buttery smooth engine, a seat designed for rider AND pillion in mind (not great, but good) compared to the cardboard seat on the SV... no comparison. Fuel economy is low 50s MPG average, and that's usually upshifting around 6-8K rpms. SV650S red-lines around 8.5K, must be crazy to always shift there.
FZ1: was afraid to sit on it even, due to the size and weight of the thing. Plus I didn't like the bazooka tube exhaust nor the 35 MPG rating. I could probably squeeze 40s out of it, but the range is still quite modest, well under 200 miles per tank. Insurance would be higher, running costs higher, higher risk of accidently twisting throttle in a turn going over a bump and getting kicked off and the required heft to move it around the garage or balance it at low speeds (parking lots, etc.)
If I had to choose again, I'd still get the FZ6. Hands down, the best bike I've ever had. If I could afford it, I'd go for the BMW F800ST. But that's another discussion entirely... hope this helps.