Depends how long you are going to keep it and how carefully you fermented it. If you are going to drink it young and were a bit 'rough and ready' with FG and priming it's plastic all the way for me. If you plan to keep it for some time and were careful about fermenting right out then priming precisely then glass is best. I say this because over time plastic will lose pressure. Anyway I think elderflower champagne should be drunk young to enjoy the bouquet at its best
I use old prosecco or champagne bottles that I bought some swing top stoppers for. They're great for popping when you have a bottle, adds to the 'champagne' feel. Seems to retain its flavour no problem in them.