Don't have a Portofino, but do have 2 other Marin roadies - a Treviso which is all steel (columbus neuron I believe) and I got it sans fork, so I put on a CF fork. I LUV that Treviso! Its just a great ridin frame for just about any kinda ridin. It gets way more mileage than any other bike I own, which includes a CF Spec Roubaix.
Based on the Treviso, I also got an Argenta. The 'plan' was to send it to NJ, where I;ve spent a full month each year for the past 10 years...
That didn;t happen, so I still have it here in CA and ride it quite a bit. The Argenta is an ALU frame, full carbon fork (not sure what year) and is quite quick and fun. It behaves a lot like the Treviso, except its not quite as comfortable. I usually ride the Argenta on the weekend hammerfests I attend.
I'm totally into Marin bikes. Whomever is responsible for 'design' of these bikes has done a superior job.