If you plan on using an OEM Europa chassis, and stock engine position and orientation, I'd go for XP.
DM/EM is the home of tubeframe cars and the flexible Europa chassis is at a disadvantage. If you reinforce it enough, it becomes uncompetitive on weight. Remember that NO OEM parts of the car are required for Mod, though a weight break is granted if you use the stock floorpan. Mod also allows MORE aero than XP, even though you are limited to spoilers rather than wings.
XP requires the stock "frame", although nearly unlimited modifications can be made to it. A downside is that the stock firewall must be retained, and it's made of cardboard-cored fiberglass. There are a number of allowances to modify the firewall. The weight minimums are higher than Mod for the same size engines.
Re-read the rules on the classes (XP and DM/EM) and see if you really want to prepare a car to the full limit of the class.
FWIW, the Europa is legal in ASP also - though you'll need an S2 body and Twincam engine to be competitive. My pal's half-prepared S2 does well with a good driver, but it won't win against the Corvettes at National level events.
