One is a game, the other isn't. If you Invite someone, then they are always notified (non-members are emailed), and they can reply to you. If you use the LoveWeb, then you both have to correctly guess each other before any contact can be made. Also, the Loveweb only notifies current members of OxfordRomance, or those who subsequently join; it does not send out emails. So, flirt with the LoveWeb; send flowers with the Invitations !
Update: There is now also the Valentine Code which is for writing into actual Valentine's cards.