The AD7175 only has 1 ADC for 16 channels, so max rate for 1 channel = 250ksps, max rate for 1 of 16 channels is = 250k/16 (assuming no delays involved in multiplexer switching. Unfortunately sigma-deltas normally incur a switching penalty because of the digital filter). The TI device has a dedicated ADC for each channel so you get the full 64k for each channel.