Geoffrey Levens, a bodyweight deadlift is not a feat of strength. Even for a middling deadlifter like me, 1.5 x bodyweight is a warmup weight. If one understands feed-forward tension and can execute a proper hip hinger, a bodyweight deadlift is a perfectly reasonable marker for a non-athlete. (This doesn't mean that, with no training, everyone should be able to perform a bodyweight deadlift. But everyone ought to take the time to learn to be able to execute a bodyweight deadlift or heavier - heavier is perfectly doable there is no reason not to learn to do it.)
Alex, "athlete" is not specific enough. Even naming a sport isn't specific enough. In general, if one improves strength in basic lifts such as the deadlift and the squat, athletic performance improves but only up to a point. I believe around 2.5 x bw for the deadlift and 2 x bodyweight for the back squat have been mentioned. It's reasonable to argue that, for any athlete in any sport, if training that improves numbers up to these points can be undertaken without compromising sport-specific training, then sports performance is very likely to improve. There's a long history to support this, lots of athletes who train more with weights in their off-seasons then cut back drastically in order to have a sport-specific focus to their training as their competitive season approaches. This is true-and-true stuff.
And in this case, you almost don't have to pick one - if both your squat and your deadlift are weak, pick one and work on it, or work on them both. Again, there are so many particulars that matter here, e.g., if you squat like a powerlifter, your deadlift may go up with very little deadlift training.
There is really no generalization to be had here - except one: Strong First.
-S-