Project Euler #1 in Ruby with duration
Performing benchmarks with such small values is pretty much meaningless, you will usually be concerned with larger values. And this is how you  tipically compare the time performance of different snippets:

require 'benchmark'
require 'set'

n = 1_000_000
Benchmark.bm do |b|
  b.report("sets") do
    s1 = Set.new (1..n/3.floor).map { |x| 3 * x }
    s2 = Set.new (1..n/5.floor).map { |x| 5 * x }
    sum = s1.merge(s2).inject(0, :+)
  end

  b.report("select") do
    sum = (1..n).select { |x| x % 3 == 0 || x % 5 == 0 }.reduce(0, :+)
  end
end


Output:

         user     system      total        real
sets    0.550000   0.020000   0.570000 (  0.580492)
select  0.200000   0.000000   0.200000 (  0.207238)