Both are old, obsolete LGA 775 chipsets. The G35 is the newer of the 2 and should support newer chips. The 975X was the first chipset to support Core2 and should only work with the older 65nm CPUs. G35 will probably support the newer 45nm chips. Neither is a good choice for a new system.
You can't game with integrated graphics anyway so it really does not matter. Also if you are buying an HP or a Dell or some other OEM the BIOS are locked so you can't overclock anyway so the chipset matters even less.
HP may have had boards built to their own specs such as adding integrated graphics to a Q35 chipset.