The J10C is a medium weight multi role fighter equal to that of the Viper.
The J16 is a heavy weight, that does come in a dedicated wild weasel / growler type version as well.
Now a single engine a/c is much easier to maintain, and very economical for CAP's, but these capabilities are already being addressed by Viper & JFT.
If it comes down to which one PAF should buy, the clear choice should be the J16.
Everything that the J10 offers is available in the Blk52+ and is supplemented by JFT. What PAF needs now, is to address gaps / additional areas that need to be addressed, especially for maritime ops.
A dedicated squadron or two for the navy, considering it's long range, would do wonders.
Secondly, the J16's low flight capability is very close, if not better than that of the mirages. Being twin engine it's payload capacity is also greater. In a Growler like config, it would wreak havoc.
Just my 2cents
@Signalian &
@MastanKhan can expand on this further.