I don't think there is a "sling vs pram debate" at all.
It's not like BF vs FF where you have all sorts of studies and an NHS campaign and experts arguing about the benefits. It's just one of a hundred and one parenting choices that some people use to feel vindicated and superior.
You are going to get people in all areas of life who think that their choices are the right and only ones and who try to push them on others who they perceive as making inferior choices.
So you get some pushy sling-users in real life, just as you get insensitive, pushy BF proponents, or rabid baby signing fans, or people who think that everyone should be vegetarian, or recycle more.
But the only place I ever see "debates" about most of these things - ie people actually discussing the pros and cons, or having a go at people who do things differently - is on forums. You only have to look at this thread to see a whole load of posts about how daft sling users and sling meets are - which is a bit ironic given the title of the thread!
I've been to three slingmeets - two in one area and one inanother. Two were great. Everyone was nice and welcoming and it was just about trying out different slings and learning how to tie them properly. I was FFing DS2 at the time and no-one batted an eyelid and I heard no negative discussion about prams or formula or anything "non-crunchy". The other one was less great. Most people there were new to slings and wanting to try them out and just chat generally, but the group leader was a bit of a pain as she was very, very evangelical about various "alternative" parenting practices and kept preaching about them. Unfortunately I was her main target because I already used slings/cloth nappies etc so she seemed to assume that I would want to "do" all sorts of other non-mainstream things. I've come across her since at a standard baby class and she clearly struggles massively to fit into any mixed group and is only comfortable with people who do things exactly the same as she does, so it is clearly a personality thing, not a "sling thing".
I have only come across one other very pushy sling user in real life. I was out and about with DS2 and had got out of the car somewhere and done a very hurried tie of the sling because I was only going in and out of a shop and didn't need to faff around tying it properly. I was rushed at by another lady with a baby in a sling who was desperate to show me how to do it properly. I kept saying I knew how to do it but was in a hurry, but she kept going on about how it would be more comfortable if I did it this way. I finished up getting quite snappy with her and pointing out that this was the second baby I had used a sling for, that I didn't own a pram and could pretty much tie any method of carrying blindfolded.
On the flip side, I've been nagged constantly by people about using a pram since I first started using a sling with DS1. Complete strangers seem to think it appropriate to tell me that my sling is dangerous/going to stop them walking/bad for my back/a daft idea/not as good as a pram/delete as appropriate. My first NCT group never stopped going on about how I should be using a pram.
There are people, particularly online, who seem to consider sling-use as a box to be ticked on the way to uber-natural parenting. I've seen people coming up with more and more obscure ways of doing things and almost seeming determined not to do anything that more than 1% of the population do. Equally, I've seen people who are happy to discuss the latest limited edition bugaboo and the relative merits of fancy changing bags till the cows come home.
I suppose the point of this long ramble is that there are pushy people everywhere, but that is about how those people are, not about the thing they are pushing.