I'd say that it depends on what you like better. Lobsters are definitely faster and more relaxing, all that you need is a Lumbridge Explorer Ring (from the medium tasks) and you will do your Lobster runs pretty quickly. The catch with yew logs, is that ALMOST ALL yew spots on F2P are crowded 24/7, unless you go to far away from banks spots such as south of Varrock or near Rimmington. You also should excpect a lot of competition from high level woodcutters, since many F2P high levels do nothing but chop yews. So all in all i'd say lobsters are definitely better, the speed at which you will make cash is relatively similar but its much less of a headache.