I am opting to do the latter. Out of speculation I expect the new guns to come in if you installed the expansion so I wouldn't be able to differentiate without reading.

Also, I may finish the main game and find the dlc was discounted later on. If I do and it's still full price no loss as I see this as being a pretty decent dlc unlike EA games.

you could probably (just guessing) get some of the radiant quests, car mods, clothes, weapons, weapon mods and get the raise in level cap before the final vanilla quest

and if you were at level cap, there may be a chance of accessing DLC areas without the quests before getting the DLC quests - but none of that is particularly experience changing