You have two issues to work out:
1. You are relatively new to lifting
2. You are a 400m runner and need to lift specifically for that.
As far as lifting for strength or endurance. You'll need to do both because you are a relatively new lifter and those moderate reps (10-15) will aide in neuromuscular benefit...ie: your body learning how to lift.
As far as how to lift for strength you're thinking of straight sets of 5 reps vs ramped sets of 5 reps.
Straight Sets v.1: warmup then 200x5, 200x5, 200x5, 200x5, 200x5
Straight Sets v.2: warmup then 215x5, 215x5, 215x4, 215x4, 215x3
Straight Sets v.3: warmup then 200x5, 200x5, 195x5, 195x5, 190x5
Ramped Sets: warmup then 175x5, 185x5, 195x5, 205x5, 215x5
Straight Sets v.1: You will use a weight in which you will reach failure by the 5th set. So the first set should feel easy, you should NOT reach failure. Progression: Once you are able to complete all sets in the same weight you increase the weight. So week 1 might look like 5/5/4/3, then week 4 might look like 5/5/5/5, following week increase weight by 5-10lbs.
Straight Sets v.2: You use a weight in which you reach failure in the 1st set. You will NOT be able to reach the same number of reps by the 3rd set....unless possibly you are a complete newbie. Numbers listed for example, you simple lift until you reach failure. By the third set you might not be able to get 4 reps or 3 or whatever. Progression: Similar to v.1 but using a higher weight. Also, this technique involves more warmup sets as you are reaching a maximal weight for that rep range.
Straight Sets v.3: Realizing you will not be able to lift a maximal weight for 3-5 sets you drop the weight expecting a drop in performance related to fatigue. An advanced technique. You have to know that your previous set was maxed out to the point where you will have to drop the weight and know how much to drop to stay at the rep level you desire. Progression: Trickier. If you make it through 3-5 sets without having to drop the weight to make 5 reps, then you need to increase the weight of the first set.
Ramped Sets: Simply increase the weight set to set until you reach a 5 rep max. Progression: You just increase the weight until you reach a 5 rep max each time.
The easiest to understand and perform are probably straight sets v.2 where you just warmup to a 5 rep max and try to lift that for 3-5 sets straight and ramped sets where you just increase the load until you reach a 5 rep max.
As far as exercises go, focus on big compound movements. You can do secondary lifts, but keep the intensity lower (say sets of 10-15). So something like this:
Workout A:
Squat 3-5 sets of 5 reps
Bench 3-5 sets of 5 reps
RDL or Hamstring Curl 3-5 sets of 10-15 reps
DB Row 3-5 sets of 10-15 reps
Workout B:
Deadlift 3-5 sets of 5 reps
Overhead Press 3-5 sets of 5 reps
Leg Press 3-5 sets of 10-15 reps
Pullups 3-5 sets of 10-15 reps
Rotate on a M/W/F plan.
As far as a long term plan:
XC season: 3 days a week, 2-3 sets of 10-15 reps. Vary the exercises to some degree. Example: Do the back squat twice a week and the front squat on the third day. Do a flat bench on Monday, incline on Wednesday, decline on Friday. Lookup Deadlift variations.
Winter: Workout A/Workout B rotation as depicted above.
Spring/Track: Lift only twice a week but incorporate plyometics. Do plyometrics first, then your primary exercises. Do secondary exercises only if you have time.
Finally, there is no such thing as "most beneficial". There is only what has worked, what is working, and what will work. THAT can change. What is working now might not have worked in the past or may or may not work in the future. Your body is an ever changing machine and your one goal should be to place demands on the body that will cause changes.
Alan