I'm indifferent to "spring mix". I like all lettuces and love arugula. But I agree with others here that those soft red lettuces get slimy very quickly and ruin a whole mix. I don't buy the bags much any more if full heads are an option. I've found that the heads of romaine and iceberg last the longest, but of course are the least interesting too. We buy the spring mix typically only when we're going to use it immediately.

The last couple years we've grown a green leaf lettuce and arugula in our raised garden beds. Far superior to anything we buy at the supermarket, even if its only for a few weeks.