How can I tell bad head gasket vs bad intake manifold?

Have an '05 Pontiac Aztek with the GM V6. Started having overheating issues. Car has 73000 miles only. New radiator cap, changed thermostat, no fix. Noted air everytime I bleed valves, all hoses are good, did a cap pressure test was ok and held pressure for 30 mins in system w/ no leak. THEN I noted, when the car is running, there is bubbling in my coolant reserve, when it's running warm, the bubbling is more frequent. So I'm getting air in the system, bad gasket right? A few shops qouted me in range of $2-3k, but if it's the intake manifold gasket would that be an easier/cheaper fix? Is there much difference? It's been a good vehicle, and I've kept it in good shape I'd but it's not worth that repair cost...