Despite the weed and various entanglements, I think a body would be more likely to emerge from the drain than the river hull. Where the drain enters the river near the Whalebone pub in the Wincolmlee area, there is a metal filtration grate up that something the size of a body would never get through - and it's easily viewable from the road and pavements around there.
Sadly, if it were snagged elsewhere in the drain it's potentially less apt to be seen for a long time, if ever, unless specifically searched for because there are fewer stretches where you can walk alongside it compared with the River Hull itself.
Meanwhile though, the river Hull is tidal and sometimes quite fast flowing. Anything going in, even if initially snagged on reeds and so on, would be pulled free and washed out quickly into the huge Humber Estuary and from there into the North Sea. It would have more chance of being spotted on the way to the estuary, but depending on how fast it moves and the time of day, once it's passed by unnoticed, the odds of a discovery plummet massively.
I do not know which would be "best" from a discovery point of view then.
