I always thought the top hose was best (as you'd want to know what temp the hottest water is), but this chap on You Tube in his tutorial on fitting the said meter (WR450) goes for the bottom hose, which made me doubt my decision...

http://www.youtube.com/watch?v=QvVe1XGfp2Q

Still, he may have got the wrong hose, but it's quite handy to see it all go together in the flesh (and the tacho too, which might be handy for R/X owners wanting a rev counter).