We are serious.They both have the same amount of data being displayed, but the 'i' stands for interlaced. You see, a interlaced picture displays the picture by showing the odd number of horizontal lines and then the even numbered ones on the screen at a rate of 1/30 of a second. The 'p' type stands for progressive that displays all the info at the same time. So is there a difference - yes; can you see it - probably not anymore.
The main reason you cannot tell comes from the type of HDTVs that are on the market these days. LCDs, DLPs, SXRD/DILA/LCoS, and plasmas are always a progressive type TV. Only CRTs can properly display an interlaced signal. The other type of displays will take that incoming interlaced signal and display it in a progressive type resolution like 720p or 1080p. There is of course a good amount of technical stuff behind that (see Deinterlacing and telecine) but we promised John and everyone else that we will keep it nice and easy.
[Thanks for the question John!]