It's 5515-X and 5516-X.
The 5515-X is a slightly earlier model and generally the 5516-X is about 50% faster for less cost. So I guess that would make it "better" by most criteria.
About the only things a 5515-X can do that a 5516-X cannot are:
1. It can have an expansion interface card added, including SFP (fiber) interfaces.
2. It can be obtained in a DC power supply version.
See the product data sheet for full details:
http://www.cisco.com/c/en/us/products/collateral/security/asa-5500-series-next-generation-firewalls/datasheet-c78-733916.pdf
