it really depends on your content. this site has a so so DA of just under 50 with over 400,00 links from just 200 something domains.
http://www.9slides.com/Talks/Growth-Hacking-is-not-sexy
this page PA of 0.
http://www.9slides.com/Talks/Fast-Mobile-Development-with-Apache-Cordova-PhoneGap-and-Backbone-js
PA of 22 since april
in comparison, slideshare has a domain authority of nearly 100 with 64 million back links from over 300,000 domains, but you would still find PA 0 slideshows on their platform as well.
http://www.slideshare.net/FrostSimula/10-tips-your-development-partner-wants-you-to-know
so you see, one is definitely more popular or much more established than the other, and in the end that plays some role, but YOUR CONTENT is what determines its PA and popularity, not the domain it is on really. too many of slideshows on both of these sites have horrible PA which means no1 cares about them or creators did not do a good job of promoting it. You have to face facts, the more niche, and the less mainstream type of content (the more professional and highly focused crap) the harder the challenges.
in comparison you can check this one on another provider
http://prezi.com/id1nlqcvqhrw/why-you-want-to-hire-me/ this content was cool and a bit aged, so you can see how it got that nice PA
http://prezi.com/a_4dfvuzpeg1/the-decade-of-data/ this content is too new.
both are on mentions of home page
http://prezi.com/isuuq2i9awi3/prezi-3d-template-by-sydofr/
here is another one with a higher PA. SO TIME IS OF ESSENCE when you are trying to analyze these choices, time and quality of content are about your higher determining factors, although your platform of choice can play a big role into how popular your content becomes and how fast, how sharable it is. not to mention the mere factor of traffics for each of these sites.
here is some hint as which platform gets more visitors overall
Prezi Alexa Traffic Rank: 1,648
9slides Alexa Traffic Rank: 239,923
Alexa Traffic Rank: 44,706
NOTE: Alexa numbers are only a hint, not really too accurate, but you can possibly arrive at an educated guess here about the correlation between PA of your slideshow page (lets say you have the same identically popular and quality content posted on each one of these platforms, at the same time) and how popular the provider itself is.
Given all of what I saw in this very loose and quick analysis, I'd pick prezi (you need a tad bit more skills on this platform) and i would spend a great amount of time and effort in ensuring what i do resonates well with target audience and would further make sure my target audience gets notice of this even if they do not naturally find this stuff on any of these platforms, meaning i would leverage as much controlled exposure to that link as i could via every available medium up to and including offline mediums such as TV or Radio or magazines, not to mention the plethora of apps and various other online mediums.