Hi,
Personally I would recommend the I5 as it is much more likely to be friendly towards CAD programs, however if you're serious about your CAD design an I7 is an absolute must. You may also find with larger assemblies you may run out of available memory so I would recommend installing an SSD and the creating a page file which can be used. Also, The video processor is very important when it comes to CAD especially on laptops where things work at 1/3 of the desktop performance. Because of this it's important that the laptop you pick does not have processor integrated graphics, instead having an on board video processor such a 920M