I think (I've only known one parent with a child with down's so I could be wrong)
one of the big differences is that parents are given quite a lot of support, from early on if they have a child with down's - where with ASDs it's not spotted that early and it's quite often a long time before diagnosis
so there's a long period of time where there's something wrong, but no-one's sure what, it's quite often blamed on parents (as in there's nothing wrong that...wouldn't fix) it's not visible and you can't do a definitive test for it - all of those cause quite a lot of stress
I don't think there's ever anything written about the positives, is there?
I consider us pretty lucky in that he's high functioning, very verbal, lol
he's harder work than his NT sister in some ways, but actually some things are much easier, he doesn't lie, he can't, lol, he's overly sensible - I never have to argue with him about why he should wear a jacket for instance
he can't understand why other teenagers muck about in class or smoke or drink, so I don't have to worry about that
as long as I have a valid reason for telling him to do/not do something he will, he doesn't strop about complaining that it's not fair as long as I give him the logic, lol
he knows loads about obscure stuff - ok you have to put up with the talking at you about it, but some of it's massively useful, lol
he can be very unintentionally amusing and doesn't mind in the slightest that I giggle away while I tell him why 
he's got a really admirable strength of character, yep he's stubborn and that's usually a negative, lol, but when he wants to do something he'll do it - even with all the barriers he has, he'll not give in to things (he has co-ordination problems and a speech disorder as well)