Well the harrier can engage jet fighters in a dog fight, and is actually devastatingly effective at it. Attack helicopters are generally 'easiy pickings' for jet fighters. It can fly much much faster than an attack helicopter, over longer ranges, carrying a much heavier payload, fire ranged radar guided air to air missiles, etc. drop gps guided bombs... it does 'everything', and it does if from a small ship or rooftop or clearing in the jungle. It's not the best at anything, but it does everything. And the marine corps have nixed any plans to replace it, leaving the JASP development program.
The thunderbolt/warthog is a counter insurgency close attack plane. Actually it's mission profile, and price point, is much more similar to an attack helicopter. it gets in close, and fires short range ordinance at things point blank. The only difference between it and an attack helicopter, is that the warthog could stand a small chance of coming out of a jet aircraft dogfight in one piece, it has a longer range, and it carries more ordinance... at the tradeoff of requiring a runway. It can't seriously engage other jet aircraft which could likely destroy it from beyond visual range with ease. The a10 and attack helicopters need 'air cover' from faster planes with real radars that cost many times as much.