According to my sources Mexicana and Aeromexico will order together about 50 717/737 or 32X to replace the 727 and DC-9. I don´t know who has the lead but Mexicana has some 320s in it´s fleet.
For the DC-10 replacement I´ve no information but it will be a descision between the 767-300/-400/777-200 and the 330-200/330-300/340-300. I believe they will go for Airbus, manly for commonality reasons. But this was discussed often enought before under several other topics. To see other rumours please visit http://surf.to/orders and http://fly.to/rorders.
Hope I helped you a bit.