You may well be right about finding direct evidence of past or present life, and underground (one hopes not too deep) is probably the only possibility. As for boots on the ground, I suspect that's going to happen somewhere along the line, although getting people to be willing to cough up that kind of money will take a motivating factor. Evidence of life gathered from a rover? Proving that we're more awesome than the Chinese, Indians, or whoever else might be flexing space muscle? (that trick worked last time...) However, I'm not convinced boots on the ground is as big a technological win or as much bang for the buck as the development of really capable autonomous robots. They'll be needed anyway for exploration of the moons of the gas giants, which AFAIK are still the best hope for life in our solar system.
(what, no dinosaur bones? )