uChess is related to this site - but only because the it has been developed by the same people. (I do most of the coding work here and on uChess)
They are (or will be) run as separate entities, but the relationship between them will remain - you will find many of the regular players on uChess are subscribers here.
As for if you should play at uChess or yahoo, that is entirely your choice, but I hope a friendly community of live/blitz addicts will develop around uChess in the future. I think it compliments the correspondence chess of this site quite nicely.