Really, given the time elements, It really only makes sense to fly within Italy unless you are doing Sicily, and maybe Sardinia. Naples to someplace near the Dolomites, you might as well take the train.
But to answer the question, both are similar, the secret to budget airlines is to know what your travel requirements are, especially as related to baggage size and weight, and buy what you need. Forget some cheap initial fare, price what you need and compare to other options. For RyanAir, just take the Plus option, gets you a 20 kilo checked bag, airport check-in, and a reserved seat, everything you need, still a cheap price.
Also, maybe neither budget airline may fly every route every day, so sometimes schedule trumps choice of carrier.