Both the S2000 and the YS-01 have the same output (GN20 on land). Recycle time w/ NiMH batteries is a bit better w/ the INON (1.5s vs 1.9s). Angle of coverage in degrees is close: 95 H/105 V for the S2000 vs 100 H 100/V for the YS-01 (w/ it's diffuser the S&S gets 110x110). Number of flashes per is 450 for the S2000 vs 330 for the YS-01. Both trigger via a fiber optic cable. The S2000 is smaller and lighter (though UW it is less buoyant than the -01). Cost difference, depending on where you buy, can be as much as $100 more for the INON.
The YS-01 has a built in aim light which, say on a night dive, can be handy. I doubt you would need to upgrade w/ either choice--except to buy a 2d one eventually. Think a bit about where things might lead for you. If you figure to stay w/ a point & shoot either is a great strobe. If you eventually would go for a dslr they might be a bit underpowered, lack hard wire connections etc. You tend to hang on to strobes and upgrade cameras, so whatever strobe you buy should last many years--and either brand would.
Personally? I like Sea & Sea. A lot. They make good, rugged strobes and keep it simple. I have a 110A and a YS-02 (same as -01 but w/out an aim light). If budget is a factor consider the YS-02. Same specs & strobe, just w/out the aim light. (It's about 100 bucks cheaper than the -01.) INONs are great, I just happen to lean towards S&S. Oh, here is a great way to compare UW strobes, courtesy of Digital Diver:
Underwater Strobe Comparison Guide - Digital Diver Network // ww