I wonder if the main approach should be constructing one's repo to have the preferred file type. Or are you envisioning a repo with a summary in bz2 and also gz, and packages presumably in gz, and some users want one and some want the other? An option to prefer a compression type when finding files doesn't really seem objectionable, but it seems unnecessarily complicated if there's a better fix of buliding the repo differently.
Do you mean just remove / don't make .bz2 for, say, m68k and VAX? That'd work, but there may be people with faster machines that have less bandwidth and wouldn't prefer that. Likewise, pkgin supports pkg_summary.xz, too, and we've already seen Alpha machines run in to issues because of overly aggressive use of high xz compression values (xz -9 makes archives that require 65 megabytes of memory to decompress), so this'll likely be an issue for low memory machines in the future.
Changing the order should be simple (reverse the order of suffixes in sumexts[] in summary.c).
John