For me, it was accidental - I had only been able to get tickets for Saturday until a last minute return became available for Friday, which would have been my first choice. Accurate recollection of two back-to-back performances does cause me a bit of confusion, and I doubt I would do the same again through choice! Yes, there were some interesting differences to note (which other posters have described very well) but for me, the positive aspect of the double experience (being new to Giselle) was reinforcement of the similar elements - second time around, the musical themes (which repeat brilliantly throughout) were more familiar, and being able to anticipate some of the action helped me to get more into the emotion of the story on Saturday.
Thinking back, common aspects of the two debuts are now giving me recurring goosebumps... Francesca and Yasmine beautifully skipping out of the cottage, to applause, into their new roles... Giselle throwing back her necklace when she learns the truth....time standing still, for the the whole company, with Giselle static, head in her hands...Berthe pushing Albrecht away as she cradles her dying daughter...Myrtha's first unwordly bourrée across the full width of the stage.....the Wilis' strange, progressive sequences of hopping arabesques back and forth... the Wilis huddling together and then parting to suddenly reveal Giselle raised up as a lonely veiled spirit...Albrecht unalbe to catch Giselle's spirit as she drifts past him...Myrtha and the Wilis cowed by Giselle, in front of Albrecht, in the form of a cross....the Wilis (26, I believe) all prostrate in honouring Myrtha...Giselle floating across the stage in Albrecht's arms...dropping flowers into his hands..... Giselle's final parting and descent into her grave..
Oh and just the way ballet dancers can move their arms so expressively, and emotively, with such slow control at times....
All just so poignant, so beautiful, and so fantastically danced and acted by the whole company.