In my opinion it's better to buy bitcoin in order to qualify for this fork. Bitcoin price will not be affected by this fork price. On the other hand zcl price will drop. Also, there is no guarantee that combine zcl and bitcoin private combine price will be higher than zcl price now. So it will be gamble for zcl holders and free money for btc holders.
If you are already holding BTC, or purchasing BTC with other alts, the fork really is like free money. However I think if someone is buying with fiat, or wants significant upside, then the reasoning changes. You'd have to use much more fiat or alts to buy BTC to get the equivalent BTCP you'd get buying ZCL. And for people with limited funds I think it is probably much better to buy ZCL for the fork than BTC. Say someone has $10,000 to invest. The upside if they buy BTC for the fork is only 1 BTCP. Right now for the same amount of $10k they could buy almost 70 ZCL. So while there is more risk the upside is potentially much better with 70 BTCP. So in that case I think it would be better to buy ZCL to qualify for the fork than wasting time moving $10k around to get 1 BTCP.