Tidal curves are well catered for by Ayetides, and I've been using that excellent app for some time now.
However there are two significant features of Navionics for iPhone that iNavX in conjunction with Ayetides doesn't yet offer, and in my opinion should do.
Tidal Flow
For me, sailing around the Solent, tidal flow is a major factor, and can make the difference between a 3 hour sail and a 5 hour sail. Having this information accessible from the charts themselves is invaluable. Before I started using Navionics, I had a local download of an animated gif that showed the complicated tidal flow hourly after HW. This meant opening Ayetides, finding the time for high water, switching to another app, working out what the time difference would be for where I anticipated I to be at that time, and then peering at low resolution image with red arrows on it to come up with current speed and direction that was at best an average of the whole lunar cycle. With Navionics, I click on an icon, drag the timeline and it tells me the flow and direction with surprising accuracy.
Keith Bater's Solent Tidal Flow diagrams (what I used to use)
Integrated system
It is logical to have chart and current information in the same place as GRIB forecast information as these inputs allow accurate calculation of your route. This saves jumping between apps which can be annoying and unnecessary. One of the reasons that people pay the normally steep prices for Navionics cartography is that it provides an integrated environment. (however, I understand that this is not the case for S57)
I can only talk from personal experience when I say that for my sailing requirements, integrated current display comes higher on the list of priorities than feature such as GRIB download, instrument integration or AIS. Therefore I currently use Navionics on my iPhone. However, if iNavX were to offer these features, then I would certainly switch back to get those other useful features.
Lastly, I would rinse and repeat this whole post for MacENC too. Now that it has support for Navionics cartography (and extremely well implemented if I may say so), go the whole hog and support tidal flow and current. Please bear in mind that users are actually paying Navionics for this data anyway.
Please take this as genuinely positive. I'm a huge fan of MacENC and iNavX. It's just that I've been bashing on about diplaying tidal flow data for years.. now that we have decent cartography that provides this data, it would seem illogical not to implement it.