first off the x55d I don't think exists. I can only find an x55a and an x55vd and neither option is good as they sport pentium dual core chips and nvidia 6 series graphics. out of those two options you would want the K401UQ because it has an i5 cpu and 9 series nvidia graphics. much much better.
and for a recommendation based on your budget I give you these two lists to get you started.
http
/pc4u.org/best-gaming-laptop-under-600-dollars/
http
/www.touch4laptop.com/gaming-laptops-under-600/