I think the keystone style gives you the greatest flexibility (ie up to 6/8 keystones in 1 plate)
I've used them throughout my house combining phone/cable/network into 1 simple location...
Edit: After sobering up, I reread the post. The difference between the decora with the rectangular inset look, and the keystone version mostly purely fashion.
1 other point, the decora max at 6 keystones, while the plain style allows 8
The company I work for used to use just straight keystone wall plates. Now we use the decora style keystone plates just for cosmetic purposes. Looks better in my opinion, plus you still have the flexibility of the keystone jacks, which means you can have up to 6 different jacks in one wall plate.
Cost is another factor.....the decoras are often lumped with several together. Price of a single decora with 2x jacks is often higher than 2x individual vanilla keystone styles.
And if you have to to replace one down the road...you often have to replace the whole decora insert, not just an individual RJ-45 that went bad. Granted this doesn't happen often...but just this week I had to replace a single jack on a 48 port patch panel. The guy who installed it used a Hubbell that had 12x per block....so to replace 1..had to replace the whole block with 12. If it were individual keystones..would have been quick 'n simple.
I use Leviton keystone jacks and plates, unless I am matching to another manufacturer's parts, or if the customer requests something different. The only time I have had to use Decora is for residential work.