This is based off of my experience, but I am interested to see if others are having a particular issue.
On three separate Macbooks, I have used both cases.
2010 unibody 13": Speck leather case, incase plastic
2014 retina 13": Speck black rubberized, speck purple rubberized
2014 retina 15": Speck black rubberized, incase plastic
On the 13" cases there were never any issues other than that speck would crack in more places, sooner. There was never one crack, but something more like a shattering look. Otherwise I had no issues with the 13" cases.
On the 15" case, however, it was so floppy that the rear corners would not stay on. I found that I would constantly have to bend them inward to get them to stay for a little while. I tried 4 separate speck cases and either they were too tight, too loose, or cracked.
The incase, on the 15" mounts to the bottom by hooking on to the rear of the unibody plate that screws on to the bottom. So there's no way it can come off, unless the hook which travels all the way across, cracked off.
Otherwise I've been happy with speck. The leather case for the 2010 was very nice, but flimsy. So I think speck needs to find a new way to mount the bottom case to the 15" MBPr. They also need to find a better formula so the plastic doesn't have catastrophic cracks.
So for me, I am going to vote that incase is a better case, out of the two.
Addition features between the two:
Incase has holes on the bottom. I don't know if this actually provides better heat dissipation, but I'm inclined to believe it helps at least a little. Speck does not have this.
Speck has air in the feet on the bottom case to cushion the impact when it is put down. Although incase also has this feature, it is very tight and therefore minimal. In this respect, speck wins.
Speck has little rubber bumps on the inside of the top case to keep the lid from touching the case. I am assuming this helps with impact, as well as to prevent dirt from rubbing the top aluminum and causing scratches. Incase has a bunch of tiny little plastic bumps. Definitely not as effective, though I've never had scratches with either.
I've not used a rubberized Incase, case. I don't know if they exist, but I do like the grip of Specks rubberized cases. I may stipple parts that I grab, or ad rubberized tape to the incase.
Last, Speck cases are overall flimsier than Incase. Some of you may have experienced the top cover not coming down all the way on one, or both corners. This gives the illusion that the top lid of the Mac is warped.
If you've used both, which do you like better and why? Also, has anyone had problems with the rear corners of the bottom Speck case?