Lack of options? You have to be kidding me. I doubt the thread pitch matters this time. Check out these websites
http://www.drumfactorydirect.com/index.php?main_page=index&cPath=6_59&sort=20a&page=2
http://www.gibraltarhardware.com/?fa=partsmain&sid=450&cid=102&curcat=2
It's normal for tension rods to be different sizes, not just on the actual head of the tension rod.
Visit a local GuitarCenter, or a local drum store around you and try to find a longer 6mm tension rod. They don't hold them. Now if you look at the options you listed, on those websites:
This one is sold out:
http://www.drumfactorydirect.com/index.php?main_page=product_info&cPath=6_59&products_id=5698
This one is $3 each, plus the shipping:
http://www.drumfactorydirect.com/index.php?main_page=product_info&cPath=6_59&products_id=8733
If you compare those prices, availability and quantities, yep, I call that lack of option. On a local hardware store I found the size and diameter that I need (ok ok, with Allen head), for less than 1/3 of the price.
Although I'm not as experienced as you might be, I can't see any theoretical difference between them, only that I'll be using a different key. Or is there anything else that I'm missing?
Those are just generic ones, sound percussion is the other company that make generic rods. You could get company specific ones from virtually any drum company on the market. Point is they are widely available in multiple sizes,
Some tension rods do have specific designs such as Tight screws which are intended to keep your drums in tune longer.
I suppose it doesn't matter if you use an allen head but I would find it aggravating having to switch between a drum-key and an allen wrench when tuning. I've heard some people tune by feeling or by how many times they turn the drum key, using an allen head might mess with that. Also you can't easily mar a drum tension rod head, I've seen hex-screws get marred, even stripped but never a tension rod head.
Granted, you don't tune electronic drums, only tighten the head to your liking.
Honestly though, it probably doesn't matter,
Just make sure you have a washer between the screw head and the rim.