Benayoun was a decent player but he was never a winger. He used to cut inside an awful lot and would seldom pull wide and crossing the ball most certainly wasn't his strength. He played more between the lines where he could use his "jinkyness" to good effect. Riera on the other hand was a classic LW, not great by any means (no real outstanding attributes) but generally a solid player in possession, with good physical strength and aerial ability...a major improvement on Babel or for that matter Maxi.
The one winger I would approve of us signing now is Adam Johnson. I think he's better than a lot of the others mentioned here, except Bale and possibly Sanchez (never seen him play), but including Lennon, SWP and Ashley Young who aren't well-rounded enough for my liking (though to some extent an improvement on what we already have). Ashley Young would be decent at the right price and/or if we had a target man to get on the end of his crosses and set-piece deliveries, but we don't have such a player (Llorente would be good).