Gyro System vs. Active Fin Stabilizers?

Recently visited a new Ferretti, engineer said the gyros made good load banks.

"The question is, which system is better at what speeds - at anchor / zero speed, low speeds and higher cruising speeds?"

For your size of yacht (I'm assuming semi-displacement ?) the correct Gyro stabilisers will be very good at anchor if specced correctly, but their effectiveness will be less noticeable at higher speeds. IMO, the latest generation active fin stabilizers from a couple of manufacturers can equal the at anchor performance of a gyro set up...generally fins will work better at higher speeds, but this depends on correct speccing etc etc.

"Are they pretty 'fragile'?"

Most Gyros I've seen are built like tanks...although mountings and dampners may be their weakest point.
Fins on the other hand stick out through the hull, so are obviously at risk of impact / high dynamic loads. Some are designed with some attempt to create a break-off point in case of an impact, others are not.

"For either one of the system, would one need 'special' reinforcement or strengthening of the hull? I am concerned about the forces that the system will impose on the hull itself."

Gyros require secure mounting to hull structure, not unlike engine / gearbox mounting. As your hull is composite, and assuming your tanks are under the lower accommodation (?) then the most likely position for gyros would be between or straddling the inboard engine beds. Its worth mentioning that gyros are large heavy units, packaging may be a challenge if not designed in at an early stage.
Fins on a composite hull will require carefully considered local reinforcement, both of the hull shell and also adjacent frames / webs. Ideally, the internal fin head will be in a water tight coffer-dam type arrangement to offer some level of safety in case of a severe impact. The fin head position is normally (not always) under accommodation, so thought should be given to accomm layout to allow hatches for maintenance etc.

"Is there more drag caused by the fins and can it have an impact on speed?"

Yes, but minimal. Allowing for a loss of a knot wont be too far out.
There is obviously some penalty to pay with gyros too, additional weight and sometimes the need to up-size a genny or two.

"I read and find a lot of information about active fin Zero Speed stabilizers but almost nothing with regards to gyro anti-rolling devices. Is it because the latter is no good? I know that Ferretti yachts have gyro anti-rolling devices in their yachts, which is built by Mitsubishi. They must surely be using it because it works, right? But is it as good as they show it in their advertising...where there is this Ferretti that is rolling so much and comes to a relative 'stand-still' after its turned on (the gyro)?"

Gyros have been with us for some years now. Mitsubishi is a major player on larger craft, but for smaller yachts have decided to go exclusively with the Ferretti Group. You may be able to deal with Mitsubishi as a one-off build, but not if its a series production build. There are other manufacturers, but I have no immediate knowledge of how well they work.
As to whether gyros work at all, oh yes they do, especially at anchor....BUT, they are heavy, large, generally need quite a beefy generator (or at least a generous share of a generators output) and need a sound shield and good isolated mounting. Depending on the roll characteristics of your hull, you may need at least two units. 'Over stabilising' with gyros can induce a rather uncomfortable quick correcting motion which in my opinion is worse than the movement they are installed to correct in the first place....application depending obviously.