I own a Rabi, and have also used a RA. I used both before I bought my Stealth 120, it just worked better for me. If you've narrowed your choices to just these two guns, make an effort to try both before deciding, you will feel better about your purchase.
But generally, your decision depends on what kind of spearfishing you are going to be doing. Since I, like most people, can't own 20 different guns right now, I try to choose a gun based on a typical day of diving for me. I like my Rabi because its comfortable to use, powerful and accurate. I know I can rely on it to shoot straight and hold up to abuse. So when I can only take one gun, I use my Stealth.
As for the Gabbiano, it is an AWESOME gun. I like it more than my Rabi for laying on the bottom and shooting fish that only grow to meduim or small sizes. It is a dream to handle, I've never used a gun that is this easy to dive with, track, aim etc. It is very durable, too. I have dragged mine all over rocks and such while diving from shore in Japan, and it still looks great, and shoots so straight! Rabispearo used it a few days ago, and while the diving conditions were "less than ideal" as he says, he still had positive things to say about it.
But, your decision should be based on what your typical day of spearing involves. If you want a great all-around gun that you will be real excited about, get a Rabi.
BTW Mikko, I finally mounted the reel, its sweet. The shooting line was wearing out (I guess thats a good thing, it means I'm using it!), so I just tied the dynema straight from the reel to the shaft. It works fine, and is a very simple setup. It also allows me to vary how many wraps I use, depending on vis etc. Anyway, thanks for setting me up with this, by far the best spearfishing-related purchase I've ever made...my fins are a close second though!