I would get whichever one fits better. If you are able to get a good fit with either, then I would go with a ScubaPro EverFlex. I believe the EverFlex is warmer than the Definition. The "fleece-like" lining in the Definition only covers the torso area. The DiamondSpan in the EverFlex covers almost the whole body. I'm not convinced that the "infrared" mumbo jumbo (in the Definition or the Bare suits) really is any better than, just for example, the DiamondSpan in the EverFlex. But, I DO believe that having a bigger area of coverage of the fleece inside the suit does make a difference.
I would choose either over the Bare just because of the number of issues I have heard of with the Bare zipper. I asked one of my shop owners what he thinks about the reports on the Bare Reactive zippers. He has at least one Reactive suit of his own. He said that the problem is that the Bare suit is so stretchy that if you don't zip it up very carefully - and especially if you try to zip it up by yourself - the zipper tends to catch on the flap underneath or something and it starts tearing stuff up. He said if you always get someone to zip you up and they do it with just a bit of care, then it would be fine. When I dived in Hawaii 2 years ago, one of the DMs I went out with had a Reactive suit and his main zipper was broken. He was the first one that told me about zipper problems. He said a number of the pros at his shop had gone to the Reactive suit and a bunch of them had had problems with the main zipper. He had gotten his back together so that it worked as long as you didn't zip it down all the way. Then it would come apart again. Sucked for him that day because somebody went to help him zip up and pulled it apart. He dived for the day with his back unzipped.
All my wetsuits are ScubaPro EverFlex suits, but mine are older and don't have the DiamondSpan lining like the new ones. They have good seals at the wrists, ankles, neck, and behind the zipper, though, and they fit me well (though they DO run small - I wear XL in everything else and XXL in ScubaPro), so mine have been very warm, for me. I am generally comfy in my EverFlex 3/2 suit down to about 70F water temp, for a day or two of single tank diving (2 per day).
I also have a pair of the new ScubaPro HD 6.5 boots with the DiamondSpan lining in them and they are really warm and comfy. I kind of wish my 5/4 suit would give out so I would have an excuse to get one of the new ones with DiamondSpan.