You'll want a minimum intel i5, and probably 4-6gb ram. Number crunching is cpu dependent, and the faster you get the better. An i7 would probably be best, but not to the extent of the added price. I would try and get an i5 if you can, if not get an i3. Make sure its hard drive is large enough for what you want.
But most importantly, make sure you go to a best buy or something, and try the laptops out. Make sure you'll like working on the keyboard and such. I mean, if one gives you better performance but one is just a pleasure to use, id go with the pleasure to use.
I have seen the flex and Lenovo has good keyboards on most their laptops. The flex is either i3 or i7. On their website its a $150 difference between the i3 and i7, which I would be hard pressed to not spend that little bit more for the huge performance increase.
The asus can have i3 i5 i7 and each with differenct ram and hard drive options. Cant find actual price new on them. But they look pretty solid as well. So I would recommend going and trying them out, see which you like better.