Eibach Pros all the way.
My 06 HHR is going to be modded starting next week over the winter and I plan to be finished by April.
But as for lowering springs I have a local HHR guy who has used every brand of lowering springs except the Canucks which I have no idea who makes them. He had the Goldlines, Pedders and then the Sportlines. He had issues with all, mostly noise issues but also bottoming out due to having over-sized aftermarket wheels and tires. He bought the Pros earlier this year and loves them. I drove his HHR a few times in town and on highway and it rides great little noise, less then my stock HHR. So I have to assume the Pros are good to go so I ordered them last Friday for 199.99.
Some vendors and members here who can afford almost any mod suggest the coil-overs. I did have these in my suspension budget but I just can't justify the expense being there are very few reviews of them on the HHR. I think the ability to adjust them is awesome but not for 6-700 dollars more then new springs, shocks and struts.
It's Pros for me. Will post pics and do a review when I get them installed in a few weeks or sooner.

 

x 100 on the Eibach Pro's! Why would anyone listen to a vendor in reply to purchasing a set of Pedders! The vendors statement is if you have the same issue that others have encountered they will be warranteed! Who wants to take the chance! I like to install something and be done with it, not hey they only take 45 minutes to change out! My time is to valuble! The Eibachs might not be as low as alot of offerings but I can say that I haven't read or heard of any issues with the pro spring! Avoid the sportlines as most have said!

Best-O-Luck ,