@Snooky97 will probably disagree with me (sorry bud!) but I think that Evans would do more for our defense. I really like Carter, but I see him as more of hybrid LB / EDGE player, and we just added one of those last year in Derek Rivers. Rivers is significantly more refined as a pass rusher, while Carter is more experienced in space, but they'd have some overlap. Don't forget that Shea McClellin is coming back, too, and he occupies the same general space in the scheme.
Evans can be a true ILB / OLB player, who can also occasionally line up and play on the end of the line, whether standing up or with his hand in the dirt. I think he's almost identical to CJ Mosely, right down to wearing #32, and Mosely has been a three-time Pro Bowler since he was drafted a few years ago at #17 overall. Look at their college tape: the role is similar, their size is identical, their speed and play recognition look comparable -- if anything, Evans seems to close even faster.
I try not to get too tied to a prospect, since there's such a low likelihood of the Patriots drafting my guy, but Evans just seems like a natural fit. Now, he may end up testing very well, and his market value will go up until he's drafted in the same range as Mosely; if that's the case, then we'll adapt, and I'm sure the defense would be fine. I'm not worried about value just yet. Evans is a great scheme fit though, as far as I can tell, so he's high on my list.