For gaming computers, using the same budget, desktops will always be > laptops.
Between the two desktops:
The Acer you're paying a premium for a processor you won't utilize the full potential of with gaming, so essentially wasting $. The video card in it is not terrible, but considering this is a gaming computer you can do better for the money you're paying out.
The iBuypower has a less powerful processor, but gaming is more video card reliant anyway, so no big loss. The video card is much better than the one in the Acer.
Other than that they're pretty similar, 8GB of RAM, DVD/CD burner, on-board audio, and 1-2TB hard drives.
Personally would get this one (if I still bought pre-built):
Newegg.com - CyberpowerPC Gamer Xtreme 1315 Desktop PC Intel Core i5 2500K(3.30GHz) 8GB DDR3 1TB HDD Capacity NVIDIA GeForce GTX 560 (1 GB) Windows 7 Home Premium 64-bit
Has the Intel i5 2500K, arguably the best gaming processor available (and its overclockable!), and the GTX 560 video card, which is the best of the two above.
And you still get a 1TB hard drive, DVD/CD burner, 8GB of RAM, on-board sound, etc...