First of all, I doubt a flat can be sold "off plan" and then be finished in a couple of months.
If you are due to move in before Christmas, beware the
end of year figures rushed new home! Get it professionally snagged and inspected before you legally complete!
The value of the balcony will depend to an extent on the view.
But a balcony will mean more outside noise getting in than it would with a normal window, and in a bedroom that may be an issue.
In my opinion a flat with a balcony in the lounge would be valued higher than a flat with one in the bedroom.
In any event, you should. be 100% sure what you are buying and it is a legal requirement that you are given all known material information so you can make a fully informed buying decision.