Goblin Grunt has more ways of gaining armor through other support, doesn't have pest, can gain elusive with the new goblin legend, and has an additional die of attack. That all being said, I haven't been that impressed with them myself in the past mostly because playing level 2-4 creatures has been so much better for the Warlord than playing level 1 creatures. With the new goblin legend I'm tempted to try something with Goblin Grunts though.
I don't know that you can make a lot of assumptions about your opponent's book off of seeing a Cub. They may just use one to knock off guard markers, which seems to be a reasonable use for them. Seeing a Matriarch will reveal a lot more than seeing a Cub. Particularly for a Straywood Beastmaster, you would probably want to play Matriachs first, and then play Cubs later for a bear midrange strategy.