1080P vs 1080i

I purchased a WD TV HD Media Player (Full HD 1080P) from Dick Smith fro $167. I purchased it instead of the 1080i version, which only cost about $70, so I spent quite a premium for 1080P rather than 1080i. The salesperson at the shop told me that 1080P is a superior product. I purchased the 1080P but now I am starting to wonder whether I made the wrong move. I have read that 1080P is better, but from what I've read on the internet, not many videos use 1080P anyway, so I may have overpaid.

I also feel as if I have been a victim of shifty marketers because the 1080P box was massive while the 1080i box was small. When I opened the 1080P box, the actual machine in there was quite small and most of the box was just air. I feel as if the producers of the product called the 1080P "Full HD" and made the box bigger in order to justify the price premium.

This media player allows me to watch videos on a hard drive on my TV, which is awesome because I don't really like watching videos on my computer and because free-to-air TV has a lot of crap on it, and so it's important to be able to pull up some quality video content when free-to-air fails to deliver. What I find annoying about free-to-air in Australia is that especially on the weekends they show nothing but sport. More variety is needed.

Update: I know I have overpaid for the media player because (see screenshot below) I have been browsing through the internet and found a similar product for $130 instead of $167! The lesson here is that you should do your research before you shop and not purchase based on impulse. It is a good idea to look on the internet first before you head to the mall.