materials questions


I have a couple of questions that others could help me with.

1) I have a bit of 18 ga. wire and was wondering if it would
   be suitable for a 6" or 8" coil.  Would it be better to get
   some 20 ga. wire instead? (I believe that 20 ga. was the 
   generally recommended size).

2) Am I better off with HDPE coil forms that are .33" thick 
   or acrylic forms that are .125" thick?  The HDPE will 
   be about 1/2 the cost of acrylic.