No, you can't choose.
Your cash is ALWAYS used first.
If there's not enough cash to cover the full cost of the spin, the rest comes from your bonus.
As a BTW; I had pretty much given up hope of ever converting my $339 of bonus chips into cash, had almost blown the lot away and was playing Happily Ever After last night at $3 per spin. I must have been
VERY lucky & hit a win right after $5 was converted from bonus to cash, because a bit later, after I had the 'Frog Bonus' max at $54, I checked the cashier & found I had $100 in cash!
Now I can't decide what to do; cash out the $100 & lose $347 in bonus chips (the sensible option), or play on in the hope of converting more (the big temptation!)... :confu
Shall we have a vote?