I like straight edges. The rounded edges are WAY too rounded for me. I like a razor sharp top edge on my cakes so I want any dummy work I do to reflect the same style. I just *lightly* buff the corner so the fondant is less likely to tear as you get it on.
I guess it depends on your style. Do your fondant cakes have a sharp top edge or a rounded edge? If rounded I'd get rounded so you are able to replicate the design and your dummy cakes won't stick out as different. 
I prefer straight edge.
Also, before you purchase them check out this site:
http://www.guildcraftfurniture.com/cake_dummies.html
They have great customer service, a huge selection, the shipping is cheap and fast, and they can customize your sizes. I use them everytime I need dummies, and have never been let down by this company.
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