the Ace appliance is the new generation and if you take the lowest license 1/2 Gig you should pay a lower price.
You can still buy the old generation.
This would be the CSS11501.
If you need SSL offload this comes by default with ACE but no the CSS.
CSS11501-K has the SSL offload feature.
Also note the appliance has many more features which can be turned on by simply adding a new license.
So, if your site grows and you require more functionalities or more BW, you won't need new hardware.
Gilles.