I am a fan of both, for many reasons. Obviously, from a custom standpoint, aluminum is a hands down winner in regards to shaping. However, an owner who is re-building his 19-6, 22-2, or what ever can tweak the coffin area a bit to match what's available out there in the plastic arena from Moeller or Incas. One might lose a bit of tankage, might gain a bit of tankage, but there are hundreds of sizes and tankage amounts to choose from out there.
One of the aspects of fuel tank compartments Allen brought up was the almost ever present smell of fuel below decks...feint, but there. It is my opinion that the permeability of the various hoses has a lot to do with that. When I left PL, we were just starting to get into the use of "low permeation" fuel hose, which stemmed from, where else, Cali, and their CARB board. Here's a Merc tech bullitin regarding the use of low permeation hose on O/B installs '09 and later...
http://marinepartsexpress.com/Merc_Prod ... 008-11.pdf