200/4 AIS or 180/2.8 - in respect to portraits.

Hi, I usually shoot off a tripod at f/11 which both are perfectly fine.  Now and then I do the odd portrait of friends/family.  I like that limited depth of field.  Those of you who use the ~200mm FL do you use it at 2.8 and is there a big difference in dof between f/2.8 and f/4?

I kinda like the f/4 due to cost and size which I can use it more often but not sure about it for portraiture. I shoot with film and DX digital, a D70 and D2h haven't upgraded as they are still working and I mainly shoot at base ISO and don't really print posters.

Many thanks.