If you have a search through past posts, you'll find that there are a couple of potential problems using metal jacks. The way most DC plugs are made, the barrel/outside is positive i.e +9v and the inside is 0V/ground. Now, assuming you're using a metal box, this poses a problem, since your audio 6.35mm jacks and circuitboard will be grounded to the box itself, which acts as a 'shield' against RFI/interference. Thus your metal box is at 0v/ground. However, by using a metal dc jack, the thread of the jack connects to the box. This thread also connects to the barrel of the jack. Which is at 9v+. So obviously there's a problem there, and your pedal won't work. How to solve this? You could
i) Use a plastic box thereby avoiding the power issue, but leaving you less shielded than a metal box (although this could be solve by the use of a metal plate in the top of the box, or some alu foil/paint. See
http://www.muzique.com/lab/magpaint.htm.)
ii) Use a plastic jack, so that the barrel is insulated against the box. This is the easiest option.
iii) Use a metal jack but drill out the thread hole for it slightly larger, so you could put filler in there and drill another hole for your jack thus insulating it against the metal box, or cover the thread in heatshrink and the the bits where it touches the box. This is by far the hardest option.
So in answer to your original question, I'd go for the small bear plastic ones.
