I would recommend FTDNA for an additional reason; You can download your autosomal DNA file from Ancestry, and upload it to FTDNA. Then after you have made sure that your mailing address is correct in your new FTDNA account you can order the mitochondrial DNA test you are after, although perhaps it might pay to wait for the next sale. However, once you have the account you can spend some interesting time looking at your new autosomal DNA matches there whilst waiting for your mitochondrial DNA test kit to arrive! (and I recommend doing the Full Mitochondrial Sequence test)