I actually feel BETTER standing. It provides the greatest vocal comfort, and allows me to move a little with the rhythm. This is important to me, since I'm not much of a dancer. When I do acoustic piano jobs .... I get a "bunched up" feeling after an hour or so.

I am glued to the sustain pedal. It's a fault, but a harmless one. I rely on it for switching hand positions or making adjustments on the fly, as well as traditional use for damping.

Learning to use your hands for the fills was paramount for me in this standing position. I use only two pedals - sustain and harmonizer on/off. They are locked together and take up almost no space on the floor. The design of the pedals is such that they rest against the front of the keyboard stand leg (at the foot)and stay put all night with no extra help. The wires keep them in line and I never have to "fish for them".

Here are the biggest advantages:

1) visual (no contest here - it LOOKS better)
2) support - diphragm is free to contract and expand for better vocal control
3) space - it takes up less space to set up this way. VERY important sometimes (MOST times for me !)
4) cartage - no need to schlep a bench
5) mobility - allows me to move with the music .... keeps me "in the moment"
6) doubling - allows me to hang my guitar without readjusting position. Like Nigel, I sometimes leave my guitar (a Parker Fly .... under 5lbs !)on my body as I play the keys. Makes for a better switch between instruments.

**)sitting is more for a recital or concert, and then, you NEED a stage and lights and other visual enhancement. For entertaining on the go, standing puts you "above" the crowd, and lets you fit in with the population better. Someone here said:
A musician sits, an entertainer stands. It's not a concrete rule, but it sure is the norm in most shows I've seen.

Give it a try and you'll get hooked.
Scott - if you didn't have to lift in and out of a car trunk, maybe your back pain would improve? I find the convienience of the load in/load out is greatly enhanced by having a vehicle that is better suited to carrying gear. This isn't a slam against your Benz .... just a suggestion that you may be adding undo stress on you back BEFORE the job even starts. I'd hate to have to lift in and out of a trunk as much as I move around.