What type of bracket to use for HDD/SSD in optical bay? (MCE vs. OWC)
The only really important thing when choosing a bracket is to get one of the correct height.  You can get 9.5mm and 12.7mmm "tall" versions, and some machines will only take the thinner of the two so choose carefully in that respect.

Other than that, what is important is difficult to canonically state further it depends on what type of machine you are putting it in, and what sort of drive you will use etc, but I would have the following observations instead:

Open Bracket Type:


Sometimes these are simply mounting brackets, without the SATA adapter mounted in the corner
If you are running a hot drive, then heat build up may be less of an issue.  However, keep in mind that this is an unlikely scenario with an SSD, and if you are using a traditional spinning platter drive, then this type of bracket may not provide optimally tight fixing (it's not screwed in on both sides) which could be a vibration problem


Caddie type:


These have a full height end plate where the CD is normally inserted.  It's important to close off the slot to prevent dust ingress, and if you don't want to tape to your case, then sticking a little strip of foam on here is an effective barrier
Not so much for Macs which always us a slot loading device rather than a tray system, but the caddie type usually comes with a fake CD-Tray strip that you can use to block off the hole in your case, which won't come with (or fit to) a bracket type device.