A butterflied or spatchcocked chicken is closer to square than round, so you might consider a square pan if size is an issue (e.g. if, as in my apartment, a new pan significantly alters the distribution of equipment in a small storage space). Other than that, I doubt it makes much difference beyond the usual concerns about which metal and so forth. I would tend to plump for medium-low sides, so the bird steams as little as possible.
9/9/12 at 4:41pm