First a matter of definition. The iPad is most certainly HD. In fact, the resolution of the ipad screen is the first one listed in the HD resolutions table as an industry standard. It is an industry standard resolution for HD, just not widescreen HD.
Well I have tried it myself on the V TV show mentioned above as well as my own Blu Ray rip of UP in both 720P and DVD sizes. The TV show was not as huge a difference as the Blu Ray rip, but there is no doubt the difference is obvious. HD is definitely better.
As for saving time downloading, I don't see the point. It took less than 10 minutes for the HD version and about 4 minutes for the SD version. Not enough difference to worry about, given it is worth having the better quality to me. Especially when they both can begin streaming to my iPad almost immediately if I choose to stream rather than download first.
Not to mention having the HD version is nice for viewing on other HD devices.
But, to each their own.