Depends entirely on your type of hunting and your budget.
You have to tell us more.
And don't take this the wrong way as I am exactly like you - I like to have great gear - but you already have a gun, and more time in the water, more practicing your "stealth", more observations of the behaviour of the fish your are targeting those are all things that will likely have a higher impact than just going out and buying a new gun.
Unless your Cressi is extremely short, you already have a great gun. I don't suspect it has a vacuum muzzle yet, so that's one easy modification you should consider for faster, longer, more quiet shots. It is by far, the most performance enhancing modification you can make on your gun.
It will cost you about 60-100E, I think. 40-80E for the new vacuum muzzle depending on the brand (the efficiency, despite various small design differences is the same) and then about 15-20E for a new spear.
Read more here on DB about vacuum barrel guns and the difference between freeshafts and shafts with sliders. I might try to find some links for you later, otherwise just use the search function.
After Market Vacuum Muzzles
There are more options, but I would list these to consider and then give you my own, personal, opinion:
Salvimar Vuoto Air, app. 40E.
This is the cheapest and quite light, easy to obtain as it is made by a big company. In stock form, it will work with both slider and freeshaft. If using sliders, it tends to need its seals changed more than others (a pack containing three seals is about 9E).
(I think it works for the Cressi, but make sure you check with the seller)
Tomba, app. 60-65E (I think) No website
Designed and made by Tomi and Marko who both have profiles here. They can be made light, too and you decide up front if you want it for freeshaft or slider. This is not a mass production item. You will get something that very few people have in the world by a father and son team who really knows what they are doing. There is a chance, you might even be able to decide on the color. But Marko can tell you more, I can not promise you anything on his behalf
.
The system has been in use for years and you probably wont be able to find anyone not happy about their purchase. Also, it is very easy to use and I think the seals lasts a very, very long time.
Seatec Evo-Air, app. 70E
Not that great value at the price point. It is only for freeshaft but it is a bit heavy. Like the Tomba system it uses regular and easily obtainable o-rings that are very, very hard to wear out.
STC, 70-80E
The most expensive of the bunch, but very modular. Very well made, seals looks nice and more sturdy than Vuoto, perhaps (?). But the priciest of the bunch.
I have used three of these four but not the Tomba system, but I have seen enough Tombas by now, to know that it is great quality.
My personal preference would be for the Vuoto for its price or the Tomba for it workmanship, because it is amazing owing something custom made for you and because I like the idea of supporting individuals who spend a lot of time and effort to push development in our sport. The STC is definitely nice, too but despite the increase in price you wont get any added performance out of it over any of the other ones.
Also, I would probably recommend you to use a slider system and not the freeshaft. I have both, but after a few years, I am slowly beginning to prefer the slider system.
I mention the weight of the muzzles as the muzzle is right at the tip of the gun and I personally do not like to have a gun that is too heavy at the nose. But again, depending on your type of hunting that may or may not be an issue.
So, to make a long story short. If your gun is not way too short, then upgrading it will give you pretty much as good as gun as you can buy new. It may not look as blingey, but it will work great. Bar that, the best value for a new gun seems to be Salvimar's Predathor Vuoto.