Narita transfer question

Hello,

In two weeks, I will travel to Japan for 20 days. To go from Narita to the city center, I am wondering whether to use the "Tokyo Shuttle & Subway Pass" or "Keisei Skyliner & Tokyo Subway", both with 72 hours of use the subway. My question is that if I buy the "round trip" option, it would much cheaper, but nowhere sets how long is the ticket valid return. Can you help me?

Thank you very much

Best regards

Tokyo Shuttle & Subway Pass

http://honyaku.j-server.com/LUCKEISEIB/ns/tl.cgi/http://www.keiseibus....

* When you buy a round trip ticket for Tokyo shuttle, it will be valid for 8 days after your purchase.

It is very clear that your return trip to Narita should be within 8 days after your purchase/

Keisei Skyliner & Tokyo Subway Ticket

http://www.keisei.co.jp/keisei/tetudou/skyliner/us/value_ticket/subway...

*Tickets are valid for six months from date of sale.

This is not so clear, but it seems that you can use your return ticket of Skyliner within 6 months period. For detail you should ask at the ticket booth.