Not used the WonderPana, but when doing some research for an UW zoom i was thinking of buying, it was a real toss-up between the systems.
Some have better, more secure mounts, some better filters, some use custom filter sizes, forcing you into their mount - and only them as a supplier.
In the end I settled on a nice local Lee SW150 MK1 w/the new rubber light shield (making it a MkII unit) that came with a 14-24 mount. I was intending to use a different ring for the lens I was thinking of, but in the end decided against the zoom I had in mind, so the kit is for sale.
So, IF you decide against the WP for some reason, I have a SW150 kit, with rubber light shield for the 14-24 for sale - US$210 shipped in NA (Canada Post Normal Post) for the SW150 Mk1, light shield & mounting ring for the 14-24. No filters (never got that far ;>). Drop me a PM if interested.