Yes, there is.
In terms of the actual seat and PE service you receive, there is no difference (well the seat is different, but same pitch etc). The difference comes in the experience. Being upstairs makes you feel like you are in your own little private cabin and you get 2 dedicated cabin crew, so the level of service is very good.
The downstairs PE cabin is a bit boxed in between the UC galley and the economy section, and is only separated, by a curtain, so compared to upstairs it feels much less exclusive.
If it was a choice between Y and downstairs PE, I would still choose PE, because really you are paying the extra for the legroom and enhances benefits. However, you do really you want to try and get seats in the bubble to make your experience that much better.
Neil