Same price for the space and bandwidth. Will you put a Dual CPU 2U or 2 1U servers? So this question can also be seen as: Will you load balance 2 1U machines or simply combine the RAM, CPU, Disks into 1 2U, and let the OS do the balancing work.
It really depends on the use of the machine, but in General you are going to get better peformance off of just one machine. Don't forget when calculating the price that you would need to purchase a load balancer.