Eibach vs H&R lowering springs
 

What are the pros and cons between the two? I will be using the stock shocks in the meantime. The car will be used on the street and so am looking for the smoother ride and more forgiving spring rates.

Similar quality between the two . Both incorporate a stiffer/progressive spring rate over the oem set-up . Therefore, ride quality is subjective. Note of caution, however, while others may disagree, the change in spring rate may over time take it's toll on the oem struts.

Thanks for the reply Johnny.

Do you know if there's a difference between the boxster and boxster s suspensions? I've heard that the s is actually 0.5 inch lower in ride height.

Eibach...
 

... has discontinued it's Pro series for the Boxster, so you'd have to go with a used set.
Both of the sets (Eibach and H&R) are geared for performance hence, they will be much firmer than the OEM.
If you're looking for a smooth ride, stay with the original springs.
Happy Boxstering,
Pedro

Personal preference:

Not a fan of lowering springs on a Boxster. The best way to lower the car is with adjustable coilovers. Handling and bump-steer often get ugly with lowering springs only.

YMMV

Recently installed Eibach Springs. 1 inch lower.

I felt no difference in ride quality. Feels exactly the same to me. I might have felt a slight difference when going over bumps. But barely any difference.