I don't know if I should buy a
aluminum one or a royalex one.

In additon to the good advice preceding this, note:

1. Keels reduce the degree to which a canoe sideslips in a wind. They also keep
you from sideslipping on purpose in a current.

2. ABS eventually degrades if exposed too long to UV. Aluminum canoes will
thus last longer if you store them in the open sun.