Everyone tells me brightness of my strobe is what limits my sensor size. What does anyone think about a single Z330? That is $650 and one of the brighter strobes and I'd think it was enough for anything. Backscatter also has a Kraken KR-S02 used for $400. The Inon S2000 seems like the best budget choice at $365. A $285 difference from the cheapest budget strobe to the brightest doesn't seem to fit with everyone telling me not to go DSLR unless I am willing to invest heavily.
Z-330 is a very good and popular option, although I have seen reports that their reliability is not what people are used to from the older Z-240 series. I have read a post on another forum from a user who took one apart and found that it's using the same tubes as the Z-240, but dumping twice as much energy into them when shooting at full power. I have personally seen one fail for no reason, but that's a very small sample size. S-2000 used to be the go-to option for shooting macro with compact cameras - they are very small and handy, allowing you to maneuver them into tight spots, although the controls are small and fiddly, complicating operation when you're wearing gloves in colder water. However, they've been supplanted in the macro strobe niche by Backscatter MF-1. I know a pro photographer who owns a pair of Retra Pros with all the accessories, yet puts a pair of MF-1s on his Nikon D500 during muck dives. The Kraken strobe is a bit of an oddball, it's got a circular flash tube, yet an oddly narrow beam at 90 degrees, and it lacks the ecosystem of accessories that exist for Inon, Sea & Sea and Retra strobes. It does feature a very large li-ion battery pack, as opposed to the more common set of 4xAA batteries, allowing for probably several days of shooting on a single charge, but I don't see that as a big selling point.
Any other strobes to consider?
Retra Flash Prime/Pro is a very good option, but pricey. SUPE D-MAX is your choice if you want ALL THE POWER!, but to be honest, they're probably overpowered for most situations. Backscatter MF-1 if you're going to shoot exclusively macro; they're mostly useless for wide angle but brilliant for macro and snooting. SeaFrogs has a new strobe out, copying the layout of Sea & Sea YS-Dx series, but I have no idea if they have resolved the problems of their earlier model that was 'inspired' by Inon. I had a pair of those, and despite their long list of features, they had a critical weakness: when triggering by fiber optics, they worked only as TTL slaves; the power adjustment controls did absolutely nothing. If the new SF01 model resolves that issue, then it's basically a Sea & Sea YS-D2/D3 at half the cost, but without the ecosystem of accessories. Speaking of Sea & Sea YS-D2/D3, they seem to be plagued by reliability problems, so I'd avoid them.
Does everyone with 2 strobes use the same kind? I'd think a pair mismatched strobes, if anything noticeable, would provide better images. But everyone I remember diving with multiple strobes had 2 of the same. It matters, because if I buy a single S2000, my next upgrade has to be a 2nd S2000, and if I upgrade further, I have to replace both.
Using two different model strobes is almost universally a bad idea. On my recent liveaboard, a diver in my group had one of his YS-D2s burn out, and I loaned him one of my old SeaFrogs strobes that I brought along as backups. According to him, it was better than shooting with one strobe... but only just; matching the output of two completely different strobes is quite a difficult process. Sometimes you want to run one strobe 1-1.5 stops higher than the other to create some shadows on the subject, giving it a bit of extra definition, but that's an advanced and situational technique - not something that you'd be using all the time.
Note that strobes generally hold their value pretty well on the used market. Every time I see used strobes being posted on classifieds forums, they're snapped up very quickly, at prices surprisingly close to retail. Older housings tend to plummet in value, but strobes usually don't, so if and when you upgrade, you can quite easily recover most of the cost of your older pair.