I just bought a harmonic balancer from GMPD for a 94 SSEi and it was $154 with shipping. I just got a SC coupler from zzperformance and it was about $45 with shipping.
Typically, if it'* the harmonic balancer, you'd be able to see the inner portion separated from the outer. Also, it would make noise other than at idle. The SC coupler just makes noise at idle.
I would say both jobs are for people decent with working on cars. They aren't difficult but can be a pain. Also, if you put ona new coupler there are other expenses. You need gaskets, seals, SC oil, antifreeze. While you have it apart, you may as well grease teh SC bearings, and clean out the throttle body and SC housing too. Since you have to take the belts off to replace the harmonic balancer, it may be a good time to change those too.
I took some time to clean everything really well, including my injectors, throttle body, SC housing, and put on new vacuum lines. I can definately tell it flows better and I get better throttle response.