To be honest,
@Shas3 - my briefest, simplest explanation od how Metformin works is it helps our bodies work better, when dealing with carbs.
Of course it can only go so far in that regard, but personally, if comfortable trimming carbs a bit more, I'd likely want to go that way before I got the pancreatic cattle prod out. Personally, I'd rather use something to ease the strain on my system rather than creating a strain (to produce more insulin) - especially if I knew I was still producing enough insulin of my own.
If your Doc is serious about adding the gliptin, then maybe ask him is he can ascertain your underlying natural insulin levels, to see if they really need boosting or if that's just likely to add more insulin to the mix when the body is already struggling with what it can do for itself.
Not sure I put those last bits at all clearly, but that's my brain dump of the day!