to_money.format

benchmark.rb

[6] pry(main)> Benchmark.ips do |x|                                                                                                                                                                                                                                         
[6] pry(main)*   x.report("n2c") { number_to_currency(amount) }                                                                                                                                                                                                             
[6] pry(main)*   x.report("Money") { amount.to_money.format }                                                                                                                                                                                                               
[6] pry(main)*   x.compare!
[6] pry(main)* end  
Warming up --------------------------------------
                 n2c     1.024k i/100ms
               Money     2.076k i/100ms
Calculating -------------------------------------
                 n2c      9.718k (± 7.7%) i/s -     49.152k in   5.089582s
               Money     18.109k (±16.7%) i/s -     89.268k in   5.108446s

Comparison:
               Money:    18108.6 i/s
                 n2c:     9717.6 i/s - 1.86x  (± 0.00) slower