One of my legs is a 1/2" longer than the other. Should I get a crank that is 1/2" longer for that leg?

There is different opinion on that. Seems one way to handle it, but there might be better ways. I think some that actually ride with the same issue commented in this thread. Maybe they'll be back to give you some thoughts on the subject.

XRays show leg length discrepancy

If you use the same crank lengths then I'd think you'd want to size the saddle height by the shorter leg unless something else is done to correct that difference.

That's what I've done.

it depends on where the leg length difference is, and if it is functional or skeletal.
i have a 6mm leg length discrepancy that is pretty evenly distributed between my femur and tibia, and a combination of shim between cleat and shoe, and slightly different fore/aft cleat placement seems to do the trick.
adjusting your cleat in that way would seem to be the least expensive and easily modified method.
once you figure out what works, you could see if it translates to a crankarm length change instead.

Thanks, I don't use cleats.

I've never measured femur and tibia lengths, I'll have to do that.

2.. ways to do it. A) build up the pedal on one side, or B) shorter crank on that side ( buying different arm length on the left would be cheaper)

of course B foot motion is circular A is a bit of a vertical Oval Perhaps a mix would work ?

Sports orthopedist would be a better resource than us, here...



{but the cost is free, opinoins are of equal value..


/..

after thinking about this issue a bit more, i think you may not have considered the all the effects of a different length crankarm...
geometrically speaking, a shorter crankarm would result in a smaller diameter circle of foot travel for that side, which means you would be simulating a leg that is shorter at the bottom of the pedal stroke, but longer at the top of the pedal stroke. (and a mixture of the two at other crank angles)
what i think you are looking for (on the short leg side) is a method that takes the existing diameter circle of foot travel and offsets it only upward, simulating a leg that is shorter at the bottom of the pedal stroke, and shorter at the top of the pedal stroke.
this is why building up the pedal on the short leg side is the only simple/effective solution (barring use of cleats)

How about building up one sole of dedicated bike shoes with a half inch of firm polyurethane foam? Maybe a cobbler could do it to a pair of Chinamart sneakers. Use these shoes with a large platform BMX-style pegged pedals.

I had a fitting with our local high-end physical therapist/bike trainer. One of the things I wanted him to explore was that I thought I had one leg longer than the other. We talked about various ways of accommodating that. Basically, he said ignore it. Then he measured me and discovered that I didn't have legs of differing lengths. It seemed like I did because I have very uneven physical development in my torso from decades of work. That causes my pelvis to try to tilt when I'm on the bike and makes it seem like my legs are different lengths. Anyway, first get measured. Then think about it. Could be my guy is right. When I tried shimming one foot to even things out, it became quickly apparent that knee was coming up higher and that wasn't good either. So the solution, if any, might depend on whether the discrepancy, if any, is in femur or tibia.

Interesting, but I'm now wearing two different shoes. One has an 1/2" thicker sole than the other. Seems like both legs now get the same workout.