I'm gonna agree with Nate. I can not conceive of the type of strength it would take to do either feat, so I can't really say which is more impressive. Looking at it objectively, Gene's is probably more impressive because there are far more suited benchers than raw benchers, so Gene's facing a more competitive field.
Subjectively Mendelson's is more impressive, because what Gene did didn't even look like a benchpress anymore, the range of motion was ridiculously small. Also fat should not be allowed to give you an advantage in the benchpress. So I don't know in the end. They're both extremely strong fellows.