Although I have made bows from neither english elm (Ulmus procera) nor wych elm (Ulmus glabra), wych elm tends to self coppice, producing long straight branches from the base, that due to their rapid growth, have quite thick growth rings. Whats more, in the UK at least, wych elm seems to be more resistant than english elm to dutch elm disease. I would personally favour wych elm for bow staves. I intend to have a go at a wych elm long bow, as soon as I've finished, my current 6 foot yew bow.