In regards to HDV being higher risk at digital dropouts:

Clipped this from digitalproducer.com

"Walton(JVC rep): In the HDV progressive system we are using the same track layout as the DV format. There are 10 tracks per frame on DV and each track has an area for video data, audio, and subcode. With HDV, the tape speed is the same, but frames are grouped in to groups of six. Since it’s a six frame GOP, instead of using 10 tracks per frame we’re using 60 tracks per GOP. Critical data is interleaved throughout the entire GOP, so if you had a dropout somewhere—a defect in the tape or piece of dust—the recovery data is more likely not to be affected, and therefore you won’t see a defect in the recording. So, the actual dropout performance of HDV is superior to DV. It’s more robust"

This of course is representing the JVC 6GOP system. How the 15GOP sytems hold up is not mentioned.

HDV is indeed more robust, however no one's format is better than the other for or against it. Sony and Canon use one error correction unit across 16 tracks, with a wider physical area of the tape used, rather than using the DV footprint. By the math you're using, Sony and Canon offer 240 tracks per GOP, offering even greater protection. Additionally, 1080 offers scan tracks (x8/x24) which 720 doesn't offer, so you have a "protection" track there as well.
None of that matters in the grand scheme of things. It's 80% hype and 20% reality. If you have an across head dropout, with one format you're out for 1/6 of a second, the other, you're out for .5 of a second at the max, depending on whether the dropout hit an I frame or not (slim chance of that). I've had one dropout in hundreds of hours of tapes at this point, and it was forced to see what a dropout caused I don't recommend mounting your HDV cam on the handlebars and front fender of a motocross bike that is jumping fairly long distances.