Well, I'll try to clear some misinformation up from this page. When you get to class there will be 3-4 options on the board.

DFW 175, ORD CRJ, ORD 145, NY 145.

There most certainly will not be a DFW 145 option. If there is then you got extremely lucky. It's been offered once maybe twice in as long as I can remember.

Now on to choice between 175/145. If you choose the 145 the you will be in DFW very soon after IOE (1-3 months - maybe less not likely to be more)
People on here in the Envoy thread have made good arguments to choose the 145 for QOL reasons. I won't go into the details maybe you can find it but suffice it say it hinges on when we get more 175s. If those planes don't come sooner rather than later then 175 may stagnate and you will be on reserve and a junior line holder longer relative to the 145.