Without a doubt a doubt, grab the heavy-duty Robert Sorby if you plan on doing some serious corner work, it’s best not to try and make do with a chisel that isn’t equipped for the job. For frequent users that need a lighter chisel, the Robert Sorby Corner Chisel 3/8″ is the way to go. It may be a bit pricey, but the quality is well worth it.
The steel, the handle and the handling are all top-notch. Of course, if you’re not looking to make mortises every day, you can afford to take a step down and get a decent chisel for occasional use. The Narex fits this bill quite nicely. It requires a bit of work to keep sharp, but if you don’t use it all that often it shouldn’t be a huge issue and it will perform well when you want it to.