Not really. They’re nearly as similar as 1080p and 720p, really. 1080i is a vertical resolution 1.5 times bigger than 720p, just like 1080p.
The only difference actually is that 720p is a progressive scan inage, not an interlaced image. This means the field is constructed top down row by row. Once the field is constructed, it is displayed as a single field.
An interlaced image constructs two fields separately in short succession, with one field having only odd rows and the other having only even rows. They’re displayed on screen fast enough so that the image appears complete, but an interlaced image can have a noticeable “jitter” effect because every other vertical row on screen is updated slightly later than the others. Depending on the display, it can also have decreased brightness or a flashing like effect because the time inbetween both fields being displayed can be visible to the human eye.