By the time you trim the fat from cut you buy for a pastrami or the cut you buy for a corned beef, how much difference is there as far as a fat cap or marbling within the cut?
Well to start with, all Pastrami is corned beef. , a Pastrami can't be a Pastrami unless it is made from a Corned Beef. It doesnt matter what cut off meat you use, You could make Pastrami out of Eye Round or top round or whatever (it wouldnt be good, not enough fat which keeps the meat moist) but it is really about the process. The meat needs to be cured and Brined first (Corned) before it can be anything. That is why as HuskyHawk pointed out, you could just buy a whole Corned beef and turn it into Pastrami. I prefer my curing of a Fresh Brisket to anything you get in Cryovac, but it does take days to do.
As far as fat trimming lets put it this way, It depends on personal taste. I (as Im sure will shock none of you, hahah) love the fattier portions, when I go to NY delis I ask for fattier pieces mixed in my Sandwich. Whether it be Pastrami or Corned Beef. So I pretty much treat them the same way, I do find though that I like to leave more Fat on the portions Im cutting for Pastrami because the smoking part of the process really renders it in a tasty way whereas If you are just doing straight traditional corned beef that is simmered the fat simmered in water for 4 hours isn't as unctuous in the mouth. So I may trim that a bit closer. Also you kinda got to know your fat. There are like 2 or 3 different types of fat/Marbling in a Brisket, some is good fat, some is kinda gross fat especially simmered. So yes Ill cut stuff meant for corned beef a bit closer. Good question.
This is where whole Packer Briskets come into play. In my picture I did a flat into Pastrami. This is because my wife likes it on the leaner side. In the Curing bucket pic you can see that flat and you can also see the point I separated from the flat already before Cure. I really prefer the point for but the point on this Brisket was kinda weird shaped and hard to deal with (uneven thicknesses) and wouldve been a Bear to handle on smoker so I used it as Corned Beef and simmered it, Cut myself a great Sandwich out of it, and used the rest for Corned Beef Hash. THat is why I like dealing with whole Briskets, there is no waste, I use every inch of that sucker for different things after trimming.