I think it does depend on the circumstances really. My DP has 4 DCs, whereas I only have 2. I think I get a lot more of the "you're brave" type comments than he does because of that.
But I think the core issue for a lot of families is that the stepdad has got together with the resident parent - and she's the one who is in control of contact, overall parenting, etc in most setups. Which leaves the couple free to decide together how to do things in their household (or the stepdad to take a back seat and leave it up to her if he prefers)
Whereas a lot of step mums have got together with a man who isn't the RP, and is forced into a certain type of parenting by his ex. She'll be the one who's deciding schools, manners, what age to do X,Y,z, eating habits, etc. And in some cases she'll be deciding what contact her ex can have too. It's that situation that makes it hard for stepmums.
I'm very lucky in that my DP's ex leaves us very much alone when we have the kids, so overall don't feel I have a harder time than their stepdad does.
Writingblock - you say your DP's ex has said awful things about your children - but presumably she's never with your children and hardly knows them, so people wouldn't see that as a threat to them. Whereas if you said awful things about your DSC people would think "poor children" living with a woman who hates them. I'm not sure that's really any different for stepdads though - there was a thread on this board very recently from a poster who was a stepdad who was slagging of his DSD (saying she was vain and inferiour to his own DD) and he was given just as hard a time as a stepmum would have been I think.