If I had to choose one, which should I get? I don't want to buy more than one of them if the addition of the other hardly has additional improvement in power.
Looks are not a factor.

My understanding is that a HF cat alone is better than a catback alone. Am I wrong? Will the stock catback act as a bottleneck if I get a HF cat?

 

Looks/sound don't matter ? Then go with the HF cat. Cat back will open up the exhaust a bit more, but only a few more hp. If you are looking for maximum gains go for the HF cat.

 

^^ agree.. and catless would be even better..

a 3inch exhaust system would probably make the biggest difference..header back..

 

but dont think of passing a cop with the open exhaust.....

 

I think you'll get more satisfaction out of the catback. You're not going to get a huge amount of HP out of either, but at least with the catback you'll get some nice noise. Replacing both should give you significantly more hp than replacing either one, but still don't expect more than 10whp gain.

 

does stock cat with HF Cat sound ricey or sound stock? I can't make any noise, not with my latenight street-tuning activities.