For this question, since we are assuming (based on the answers) that the anime is definitely going to come to an end and therefore that there is no possibility of it continuing at some later date, which would be my choice, I think the anime should aim to be standalone in this case. Leaving it open ended causes three major harms.

One, it gives hope that the anime will continue. I think that's generally harmful when there's no possibility of that in sight, as it leaves a bunch of disappointed fans who never get to see any payoff. Two, fans of the manga will be disappointed, since they not only won't get to see their favorite scenes, but won't get to see any creativity in the translation to the anime. Three, it still confuses those who would want to start the manga. These people would not be given an indication that the manga continues, which is a separate issue from whether or not it ends at this point. Something with a "false" ending can still state that the manga continues past this point, though that may engender some confusion when scenes in the anime don't happen.

I think it's generally better to at least try to create something satisfying. This is coming from someone who has, so far, only been upset with the results than anime that have ended early have tried to produce. I was not a fan of the shift over in FMA. I was not impressed by Soul Eater's ending. Nonetheless, I would support the effort involved in showing some creativity and trying to conceive of an ending that's satisfying over essentially saying "it's not worth our time" or "we're just not going to try."

It makes more sense to end the anime properly by making a ending separate from the manga. Leaving it open ended will just make people who don't read the manga think there will be more episodes coming afterwards.