What are you trying to accomplish in this brew? Are the hops just for bitterness? If not, that kind of hop character are you looking for? Are you using any other flavorings? How roasty will this brew be/what is the grain bill?
Porter is one of those styles where you can go a lot of different directions with the hops. There is a range of bitterness that will work depending on how much crystal and/or roast character the beer has. Either US or UK type hops work pretty well for flavor and aroma (although you need to be careful not to overdo it if you're using West Coast-style hops).
I shoot for 0.6-0.75 BU:GU (although you can go higher or lower than that). My American porters use Nugget for bittering, Cascade for aroma and something else (Willamette currently) for flavor. I usually ferment with Chico. For my UK porters I use Magnum for bittering, Fuggles for flavor and EKG for aroma, and ferment with S-04 or WLP007.