I would start with bitcoin for several reasons.

1) Bitcoin has established it's place in the market (digital gold)

2) Bitcoin is the most commonly used trading pair to purchase other crypto assets (if you decide to trade)

3) The first crypto ETF will be Bitcoin NOT Bcash - when this happens I would think BTC will skyrocket.

Bcash has not established it's place in the market yet.  Maybe it wants to be the "usable currency" but that title is still up for grabs with litecoin, dash, or who knows.

Disclaimer: I own both BTC and BCH.  So my advise would be to buy both, but if you have to choose for some reason...You should go with Bitcoin.