Okay I'll give you the best answer to this question. The stock Ion barrel is really one of the more poor quality barrels out there. But it's allways up to you. If you are willing to invest say $50 US then a J&J ceramic is nice, a step up prolly a CP or Stiffy .689 or .691 bore for a a nit under $100 US.
If you have the money (only saying since you did buy a Ion this may not be the case) I love my Freak Barrel Don't need the whole insert kit just get 3 inserts .689, .691. 693 That will cover basicly all paints you may shoot and if not then you really should get different paint.
As for the length of a barrel either 12" or 14". This has been tested to be the ideal lengths. Any shorter becomes less accurate and any longer the same effect. Though some say a longer barrel will make you shoot a bit farther this is a myth.
So if money is a issue honestly get a .691 bore J&J ceramic in 12 or 14" and if not then get a freak kit or Evil Pipe Kit or similar item.