"conal" vs "conical"

Overall, "conal" is practically unused. For this reason alone, I would stick to "conical."

See Google Ngram for 1800 to 2000 (I couldn't figure out how to embed this.)

EDIT!

As the learned wfaull suggests, in math, we will invariably refer to the circle, ellipse, parabola, and hyperbola as "conic sections" or sometimes just "conics." The words "conal" and "conical" are not used per se in math - conical is used in design, drafting, and physics, amoung other subjects, but I have still never seen "conal." Ever.