In my shop is a 48" Betenbender, which is a hydraulic shear that will knock off 1/4" thickness.
So more capacity is always better !!
But, some tradeoffs, the gap/clearance between the cleaving planes needs to be set a little less for thicker material, which will cause "wiping" or bend-over of the sheared edge on thin materials like trim coil (etc).
Blade condition is going to be very important in a used shear. Some have flipover blades which have 2 potential edges, some even have 4 potential edges. But its costly to sharpen them, I haven't gone that road yet but need to someday...
Usual, look out for repaired castings, make sure the hold-downs work properly. Take some 3/16" material to shear...even if its only 2" wide.
And lastly there are always fearsome flywheel-based mechanical shears out there. Those are scary because once the clutch-tooth is tripped, the thing is going to cycle no matter what. With a hydraulic machine if you don't like what you are seeing you stop the process right there.
Shears seem to go exponentially in weight as capacity goes up, so study your rigging and transport capabilities too.
Aside - I had a 36" Tennsmith earlier in life and loved it for the clean fast cuts. I thought I could cover all bases with the bigger machine but due to the blade clearance issues I wish I still had the small T-smith due to its fast clean cuts.
