This is a serious debate ; both have their advantages and challenges.
Aluminum has a better impact resistance than fiberglass, it will deform before breaking, and remains lighter on larger structures. GRP offers phonic and thermal qualities that are often more interesting than aluminum, but tearing at impact is immediate. Alu is often preferred for hostile sailing, North Pole or major crossings. Aluminium will age better over time, without major maintenance and without risk of corrosion. The GRP will be subject to its natural aging with risks of osmosis and porosity of the surface after over ten years, and requires to do the submarine every year. Now the quality of the work also has all its importance some ; some Catamarans of over 20 years old in polyester reveal still exceptional finishes, provided you want to maintain it properly !
The known electrolysis problems on aluminium boats have been explained (phenomenon with stainless steel or other metals nearby) and have now found a solution : proper insulation, grounding, all of the solutions that make it possible to isolate the risk of electrical leakage. Electrolytic pbs can also be found on polyester multihulls, often at the level of shafts or Sail Drive (line cutters, stainless steel stock, propellers in bronze, etc.) The quality of implementation is still a must for total serenity, whether it is polyester or aluminium.
In term of comfort ?
We do not have the smells of styrene or resin, and we have a structure generally more silencious, without creaking or cracking : a rigid structure. In addition we have a great freedom of arrangement of spaces with aluminium, while GRP requires a strong structural calculation, most of the time not modifiable : we often find some inserts, reinforcements, not always very harmonious.
Security ?
Aluminium does not burn, polyester does. And we will not forget the Eco side, very current : aluminium is 100% recyclable, not polyester; it is a real society problem to know how to manage the stocks of old boats …