short answer 1080p (progressive)
1080p is better than 1080i technically, but only for certain close-rang viewing distances depending on the size of your HDTV. If you have a 50″ HDTV and sit more than 8 feet away from it I don’t think you could tell the difference between 1080i and 1080p easily.
Your TV isn’t archaic yet, just that more movies are being made in 1080p natively. If you are happy with the picture quality of your TV, then that is all that matters. Just ride out your TV till it starts to breakdown and then buy whatever the latest tech is.