I would strongly advise a fruit cage! We have built one similar to Geoff's, with wood around the base to strim against. Also we have piled it with about 4-6" deep wood chippings to help keep the weeds down.
As for your brassicas, if you want them aphid and caterpillar free, I would recommend mesh. (enviromesh or similar) I will be using it for the first time this year, as although standard netting keeps out the birds and the flutterbys, I am fed up with broccoli and sprouts infested with aphids.
It is definitely an investment, by which I mean, not the cheapest option, but I don't want to have to throw away any more PSB that is more grey than purple