PP, Lounge Club, etc. typically all provide the access to the same lounges - so pick whichever is best priced. If they don't travel much it might be better to have one person pay for a membership and pay for the other as a guest (typically around $25-30). Also worth considering is for you to get a credit card that includes PP or other program (e.g. Amex Plat) and sign them up as a co-card user. Lastly, many airports now offer paid lounges so it might be more cost effective to just pay if they don't travel very frequently.