GTX 5xx Series SLI

i m doing a SLI on GTX 550ti , becoz on tigerdirect i bought accidently a gtx 550ti worth only 70 bucks. now i realised its not so good, so i m buying a second one to SLI.

i m confused between 2 gtx 550tis , one is zotac gtx 550 ti and other one is same to my previous one i.e. gigabyte oc edition.

zotac has 1000mhz clock , but gigabyte gtx 550ti has 970 mhz clock.

now should i buy the gigabyte's which is same o my first one , both are same model. or should i go for zotac which is 30 mhz more clocked. will it perform better if combining 2 same models ?