Margins are not the issue, nor is ease of assembly. Apple has enough design experience and experience working with their partners to work through assembly problems. Besides, who cares if it takes 6 minutes to assemble instead of 4 when you're paying $10 a day?
The real issue is going to be marketing. EVERYTHING is about marketing with Apple. I don't see them selling a product solely on the basis of one component having greater specs. Rather, they will sell it on the basis of the entire package.
Look at the iPhone 4. Did they sell the phone because of the screen specs? Probably a few. But the majority were probably sold on the basis of the entire system's capability. Faster processor, more RAM, greater battery life, gorgeous screen, etc. The focus was on the PRODUCT, not the specs. If you focus on the specs, you make it easy for a competitor to produce a crappy product which happens to have a better spec - and beat you at your game.
It's not like Apple (or any good marketing company) to try to sell their system on the basis of a single specification. Rather, they'll focus on the entire package. Can I see an iPad HD added to the line to sell alongside the regular iPad? Yes (assuming the technology is available which hasn't yet been proven). But it almost certainly won't be sold on the basis of the screen alone. It will be sold on the basis of the entire capability of the device and will focus on the capabilities, not the specs. Thus, I see 'iPad Pro' as being far more likely than 'iPad HD'.