Like Willy said both will work but if you are planning one or the other I would say go with the flat.
As said the point has a lot of fat in it (very tasty though) and the flat is more what you would expect from pastrami and is also a lot leaner.
Also remember when cooking pastrami you don't need to bring all the way to 190 like you do with a regular brisket to achieve the same level of tenderness, the cure has helped to tenderize the meat already. I like to bring mine to 170 then let it rest a bit or for an even moister tastier treat cool and refrigerate overnight then reheat with steam the next day. You won't believe what a few hours in some steam does for it, sooooo good.
That's the way they do them in the better establishments in NYC.
After you get a few under your belt try curing your own beef then smoking it into pastrami.