Choosing a yarn

i was in a yarn store today and almost bought the blue sky alpaca cotton for the shawl collar scarf, but i held off because i have another project promised first. but i wonder - could i use that cotton worsted for the black pearls scarf, which i'm dying to make for myself? the pattern calls for peace fleece worsted, so the weight ought to be about the same.

i know this is kind of a dopey question, but i'm always nervous about deviating from patterns, but since i can't wear wool, i often have to.

tomorrow i buy yarn for my first scarf, tilted squares!