Ruby: String#chars.each vs String#each_char

bench.rb

#!/usr/bin/env ruby
require 'benchmark/ips'

GC.disable
STRING = ("a" * 100).freeze

Benchmark.ips do |x|
  x.report('chars.each') { STRING.chars.each {|c| c.upcase} }
  x.report('each_char')  { STRING.each_char  {|c| c.upcase} }
  x.compare!
end

__END__
Result:
  Calculating -------------------------------------
          chars.each     4.238k i/100ms
           each_char     5.022k i/100ms
  -------------------------------------------------
          chars.each     48.040k (±46.1%) i/s -    194.948k
           each_char     62.546k (±42.9%) i/s -    210.924k in   5.262923s

  Comparison:
           each_char:    62546.1 i/s
          chars.each:    48039.9 i/s - 1.30x slower

Ruby:

    ruby 2.3.0p0 (2015-12-25 revision 53290) [x86_64-darwin14]

Hardware:

    Hardware Overview:

      Model Name: MacBook Pro
      Model Identifier: MacBookPro12,1
      Processor Name: Intel Core i7
      Processor Speed: 3,1 GHz
      Number of Processors: 1
      Total Number of Cores: 2
      L2 Cache (per Core): 256 KB
      L3 Cache: 4 MB
      Memory: 16 GB
      Boot ROM Version: MBP121.0167.B15
      SMC Version (system): 2.28f7

Software:

    System Software Overview:

      System Version: OS X 10.11.3 (15D21)
      Kernel Version: Darwin 15.3.0
      Time since boot: 23:27