Two natural  vellums are used for banjo heads, calf and goat.  For banjos calf is more traditional and I have seen still sound original calf vellums on 80 year old banjos and although often over stretched, goat vellum is also used and is usually more economical. Calf has a  reputation for longevity,  goat is used traditionally on tamborines and bodrhans which indicates certain durability.

Price may affect the choice of vellum especially on less valuable instruments, vellum prices vary a great deal from the really economical Asian to the top quality European and American. As with everything there is an element of you get what you pays for. Good vellum is ready finished and only needs to be lapped (fitted), cheaper vellum usually requires some finishing after fitting and will still not equal the highest quality vellum. Use the best vellum that you can afford taking into account the value of the renovated instrument, calf if economically viable.  It cannot make economic sense to fit a £ 48 calf vellum to an instrument that is going to fetch £150, where as £25 - £35 extra on a good calf vellum for an instrument that will sell for £1000 can.