I have heard that the Unity set consists of basically the Imagine Mid with the Clarus Tweeter. I was going to go with the Unity set myself, but with the recent deal on the used images from Hybrid, I had to jump on that.
Just skim through the Unity manual (available here). The manual says that the tweeter and crossover box are from the Clarus set, and the woofer is based on the Imagine woofer. Their parameters indeed look very similar.
It seems like Unity is a better set if you need a more robust crossover box. It gives up 2nd order crossover slopes and optional tweeter attenuation. The Imagine uses first order high pass for tweeter and no option for attenuation. If you're going to run active, then the $100 price difference between Imagine and Unity is buying you a different, hopefully better tweeter. Personally, I'd spend a little more on amplification and run Imagines quasi-active.
The tweeter is a little better in the Unity than the Imagine set. However, since your asking between the clarus 6.5" and the unity 6.5", the Clarus has slightly higher xmax (7mm vs 6mm one way), slightly more cone area, higher power handling, and a much lower FS (50hz vs 75hz).
Thank you everyone for the replies I feel a little more knowledgeable on the different sets now. So if I were looking for a good front stage in a civic 2000 ex coupe would the Clarus provide a noticeable difference to the imagine running active?
My bad, I forgot this was the question about Clarus and Unity after reading the first reply. Yeah.. if you can afford Clarus and have the mounting depth for it, this is a better choice. The main difference is the woofer, which is a step above the Unity woofer. Looking at the specs, it seems like it can be high passed at around 50Hz and actually deliver the goods. I would have loved to see it in my car, but I think the mounting depth of 3inches or so could be pushing it, and the door panels are too close to fit a thick baffle.