I am going to be getting 2 young guinea pigs in a few weeks and want to have the cage ready and waiting. Should I get one layer 120 cms, 2 tiers of 100 cms or 3 tiers of 80 cms? I want to have a lid, as I will be having a foster child and it feels safer (for the pigs) that way. c & c cages look great but getting a lid adds on another £30! Thanks.