Just tossing out some averages to think about. First, you are just getting into buying gear so getting into diving and having a boat, you will most likely own several different tanks sooner or later so what you get first might not matter too much.
Second, you are getting into Nitrox and looking at doing 120 foot dives. If you ballpark a non-deco dive to 120 feet and you have an average consumption rate, you are looking at burning roughly 80cf of mix. If you are working or in rough conditions, that will be much higher.
Lastly, you need to really look at tank specs when you make these selections. Not all 100s or 130s are created equal. Some will be negative when almost empty while others can be positive. Some 130s can be the same length as another’s 100s. I prefer to dive with little to no weight on a weight belt so I go with cylinders that will work best for that purpose. Depending on location and what you want, the cylinder market (new and used) can be a bit rough right now.
Good luck with whatever you select and as I noted, if you are like many of us, you will own several anyway so your first cylinder might come down to the only size you can find that is available.
