You can fit larger parts in a clamshell blast cabinet, but if you put too large of parts you can't manipulate them without opening and closing the cabinet all the time, spilling media all over the floor.
I have a side door and I don't really feel it limits my work piece size, I can fit parts through the door that are a pain to wrestle around in the cabinet, and often have to take them out to reposition.
Like Mike said above, the clamshell design is messier and more surface to seal and a larger potential for leaks but I could see where it might be handy.
The blast cabinet we have at work actually has a top loading clamshell and a side door, I've never needed to use the top load.
Whatever you do get a bigger cabinet than you think you need if your budget allows.
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