Can't you download an SD and HD trailer? I refuse to open iTunes on my iPad or I would :-p
Not from what I can see. The only thing about "Trailer" is the "preview" button, but I dont think that changes. Not sure though.
Honestly save your money, SD will be fine on your Ipad, not worth the extra just to watch on there, now if your watching a HD on a full size HD TV then yes.
You kind of answered you own question as well, when you said, I tried the trailers and there wasn't much difference, and thats just it, not enough to warrant the extra cost.
I think the HD rentals look fantastic(sorry, haven't rented SD yet), but if you've got a 16GB iPad beware. The HD movies are like 3.5GB.
You can simply download an SD and HD (720p) trailer from any movie of your choice from Movie Trailers - iTunes and upload it to your iPad via iTunes. The 480p and 720p resolutions are what will be delivered to you when you rent.