Welcome to FT.
Right now the DFW-FRA nonstop is scheduled as a 3-class 772 for May 2016. That means First will have the fully flat Flagship Suites but Business will have the older slanted NGBC setup.
If you want the nonstop and to avoid a slanted business seat, then First it is. Alternatively you could try DFW-CLT/PHL-FRA in business on the A330s, which have a fully-flat setup.
Also, if getting switched to a slanted business seat is going to ruin your trip, I might suggest avoiding AA all together. Next May is still light years away in the airline world, and with the merger, fleet integration, aircraft updates, etc., there's a high probability that what we see now in terms of TATL scheduling is far from final.