Either is fine - Google knows they are synonyms. So do Bing & Yahoo.

You could use both if you wanted.

I do see a pretty big different in the SERP's between the two...

you should be including both, I am using variations and when checking searches they are coming either one, like TX and Texas

If you only have the option of using one you should to the external keyword tool and see which gets searched more.

I also use both. Sure, the search engines may understand they are synonyms, but the results that come up for each search are usually different. Kind of like "salon" and "salons". One letter makes a difference, and if a business is serious about wanting as much web exposure as possible, both should be targeted.

Matt

[DELETED]