You can match stripes when sewing. I don't know what to tell you for sure, but do your sewing from the right side, and don't wait till you have several stitches done to pull the stitch up snug but do it after each stitch. What you see is what you get. If the placement of the stitch didn't cause the stripes to match take it out and try something a little different.
Knitting in the round doesn't produce perfectly matching stripes, you have the jog factor then. There are remedies, but perfection is hard to acheive. I think sewing will do it better than in the round actually.
One final idea. For years I sewed up a way that is not mattress stitch and that I have never seen recommended on a site, or in a book. The woman (Verla) who used to own the LYS here taught it to me, and she learned it at a workshop she went to and liked it over mattress stitch (it makes an invisible seam too). It involves working from the right side and sewing back and forth like mattress stitch, but you don't put the stitches where you do in mattress. You insert your tapestry needle head on into the very side of the piece of knitting (the thickness of the knitting, not anywhere on the sides) and go under a thick place there (Kind of like a knot. There are places where you could try sewing into and there would be nothing to latch on to. Look for a spot that will give you some purchase.), and back up and out on the other side of the thick place. Then take a stitch in the same way on the other piece that you are joining. You are basically butting the two pieces together. It is not rocket science and is very forgiving. You try to keep the stitches the same size (or maybe it is more like sewing into each thick place on a side, without skipping any) so that you are matching basically stitch for stitch, but you can fudge things as needed as well by taking two together or skipping if needed to keep things even. I think I have used this method over mattress for helping to match stripes up and found it worked better sometimes. I wish I could show you.
I took time out to make a sample and sew it up to see if I could do it. I made two pieces with only 4 stitches and made 5 stripes of 4 rows each. I sewed them up Verla's way and it worked fine. Maybe I could have my DH take some pictures of this when he gets home, if you are interested.