But the biggest difference to me is in playing ergonomics and weight. That ES 335 sounded glorious but was a pain to play, the wide body gave me neck and shoulder cramps. The SSH is light as a feather, and fits against my body like the most comfortable strat.
I always jokingly say that playing my Custom Shop 335 was like driving a 1954 Buick. Fun for a short spin, just didn't enjoy the antique ergonomics, and didn't keep it very long. I'll admit that I also prefer the PRS tone, but that's hardly an objective comment.
Yes, the 335 has a solid center block with glued-on wings, but there are so many differences in the construction of the guitar, and the hardware, that the SSH sounds very different from a 335.
Yes, it has a semi-hollow hollow resonant peak, but the frequency of the peak is different, probably owing to the size of the 335's wings, their depth, and their materials (laminate pretty much everywhere on the top and back vs solid carved). And laminated woods behave differently when vibrated than solid woods. The hardware's different, the scale length is different...and lots more.
The Special Semi-Hollow has a tighter, more solid bottom end, the lower midrange is also tighter (by 'tighter' I mean that there's a faster, more defined attack giving less mush), and a thicker upper midrange.
It does have an airy top end, and does things the 335 can't do, with its switching options. I'd rather have the SSH. But if you truly need a 335, get a 335.