would be interested to hear what method of sharpening most of you guys use on your framing chisels, hollow or flat, and why you prefer one over the other. thanks

Hollow.

Reason number one: I own a Tormek.

Other reasons:
Honing is very quick.
It touches up fast and easy in the field.
It's easier to adjust the bevel angel on short notice.
and
Theoretically there's a less resistance (can't honestly say I can feel it).

Flat, as we use the veritas horizontal wheel.

Either can result in a satisfactory edge -- I think it is a matter of equipment and taste.

GH

I have the Makita sharpener so my chisels are flat ground. My new Barr chisels came with a hollow grind, which seems to work just as well, I can't say that I have noticed a difference between the two styles. When the Barr's need to be reground, it will be done on the Makita wheel (flat) because I happen to have one.
I have however noticed a difference in the angle of the grind. Lower angle (25 deg.)works much better in softwood than 30 deg, which holds up better in hardwoods. I have an old laminated chisel (Ohio Tool)that was ground at about 20 deg when I got it, and had big chips in the edge. It chiped easily when I resharpened it until I changed the angle to about 30 deg. Now it holds up fine. Tom

Hollow, simply because I own a Tormek. I grind my chisels at a 25 degree angle which works great on almost any wood species. Resharpening within a minute if needed.

I doubt that there is a great difference between hollow or flat. As long as it is sharp it will do the job. It might stand up longer if it is flat though. But I never tried it.
I am spoiled by my Tormek. If the chisel gets dull I can resharpen and hone within a minute.

Hollow, with a tormek.

You can shave with it when its done.

I think its the leather wheel on the tormek that really gives it the razor edge.

depends on the person,

a good chisel I think is a "feel tool". Does it have a good center of gravity? does it reverberate?
Does it need just the right force?
I use waterstones. I like to feel the sharpening. Flat. The Tao of sharpening. I can shave with my chisel too.

I'm not sure our chisels reverberate. Do you have to pay more for that?

I have used a Tormek somewhat extensively. It produces a lovely edge, but in my opinion is way too slow, especially if there is any kind of nick to be ground out. I have used diamond plates and water stones quite a bit and found them to be tedious by themselves as well. The "scary sharp" system, which relies on glass, paper, and adhesive, works well but the paper gets expensive and I don't really enjoy screwing with spray adhesive. That produces its own kind of tedium.
The system that I have settled on, which is efficient and effective, is as follows:

I bought a cheap two wheel grinder that runs way too fast for comfort (3600 rpm, I think). I then purchased a norton cool-grind wheel, hard felt buffing wheel, and diamond tipped wheel truer. I also purchased a sharpening guide from veritas that includes a clamping guide for holding your tool edge square to the wheel. I never use this last item except when I am truing the wheel with the diamond truer.
I bought a set of Norton water stones :200,1200,4000,and 8000. These stones are a bit slower to dish, and produce quick results. I adore the two finer grades. As my system evolved, the two coarser grades ended up gathering dust.

Here is how I sharpen:

I set the tool rest for my chosen angle -- about 30 degrees. I then typically free hand grind a hollow bevel, keeping a dish of cold water to dip the blade in as necessary. The cool grind wheel helps keep the temps low. If you could find an 1800 rpm machine, that would help, too. If there are no major nicks, I grind the the hollow until it is just a hair's breadth from the actual cutting edge. This ensures that there will be very little metal to hone when I move to the stones, while the hollow grind provides a positive register against the the stones surface. this is important, because I have dispensed with any honing guides -- also cumbersome and unnecessary, in my opinion. I hone first on the 4000 stone, then on the 8000 stone. I then use the felt buffing wheel, turning the wheel backward BY HAND, though it is mounted on the electric wheel, to take off any wire edge. This is a very brief final operation which may usually be unnecesary as well.
Usually, I can go many sharpenings on the stones before I have to grind, and I very seldom need to grind the actual cutting edge, so the danger of burning the steel is avoided.

This system is fast as can be, it is not fussy, and it is relatively inexpensive. Try it, you will like it. I've tried all the other approaches and this is my favorite.

Good Luck!

Pegs, you hurt my feelings. Yes you do have to pay for that. It is called taxing on your elbows.

I guess pegs's chisels are always so razor sharp that he has never felt the sensation in his hands when a chisel will not cut anymore of that oak mortise.

Unless you use an all metal chisel you are going to have a little recoil sometimes (even then it seems there would be some, but to small to notice). I guess you lack in tactile sensory perception.

But if you thought I meant reverberate in the good sense well:

reverberate: to reflect light, etc.

Every morning, when I wake up and shave, I use my 2" chisel that reverberates my mirror image, while I shave my face with my 1 1/2" chisel.

Both were sharpened with waterstones.