Well, the M&M credit card issued in the US isn't very spectacular. Occasionally they have a 50k signup bonus and much else. Reports about it are here: http://www.flyertalk.com/forum/lufth...t=Barclayscard

With a score of 600ish you may run afould with Barclays strict(er) policy of not handing out the platinum card.

I would rather focus on collecting UR points using your CSP and move them to United Mileage Plus and redeem on LH flights ex CLT. LH is pretty generous in releasing award inventory to partners. UA MP doesn't charge the atrocious fuel surcharges that may cost you nearly 1000$ on top of each mileage ticket.

Look at the Avianca Lifemiles card from USBank, that is another FFP which offers no fuel surcharge and has access to LH award inventory. USBank is also restrictive in issuing credit cards. AV LM has generous cash and mile awards and allows you to purchase miles at low rate occasionally. Of course such spend gives double miles on the AVLM credit card too